Diff fragments can now be expanded in three ways:

 - expanded above or below the current context by a fixed number of

   lines (20);

 - expanded up to the previous header (i.e., method definition); or

 - expanded completely above or below.

After a diff fragment is expanded in one direction, it can be

collapsed via a floating button that follows the viewport. If there is

still more of the diff to be expanded, it may be expanded again.
The comment_diff_fragments view now takes an optional URL query

parameter, lines-of-context, which indicates the number of lines

above and below that should be included in the fragment. Likewise,

the build_diff_comment_fragments now takes a lines_of_context

parameter and uses that to calculate the first_line and

num_lines parameters for get_file_chunks_in_range.
The activity indicator functionality in RB.apiCall (in

apiUtils.js) has been refactored into another method in the same

file: RB.setActivityIndicator. Asynchronous activity that does not

involve API calls can use this method to enable or disable the

activity indicator.
Add the expand_fragment_link and expand_fragment_header_link

Django template tags. These tags create expansion links for use in

diff fragments. These functions use a new template:

reviews/expand_link.html.
Added Brian Cherne's hoverIntent jQuery plugin to the 3rd party static

bundles.
The DiffFragmentQueueView (diffFragmentQueueView.js) now handles

the expansion and contraction of diff fragments. This view monitors

for clicks on expansion and contraction buttons and dynamically adds

a script tag which will replace the contents of the appropriate diff

fragment with the same fragment, but with expanded (or contracted)

context. The activity indicator is turned on when a diff fragment is

requested and off when the requested fragment has been received.
The expansion links for diff fragments are only shown when the mouse

is hovering over the comment container for that fragment. Once the

mouse moves off the comment container, the expansion links will become

hidden. The hoverIntent jQuery plugin ensures that clumsy mouse

movements will not result in the controls toggling on and off

repeatedly.
The ReviewBoxListView now constructs the DiffFragmentQueueView

with an element (#container) on which it should listen for events.
The diff fragment template (diff_comment_fragment.html) has been

modified to only show 2 columns (instead of 4) when the file in the

diff is a new file. This change makes the diff comment fragment viewer

look more similar to the diffviewer.
When a diff fragment is expanded to have context, the lines that have

been flagged for review will be surrounded above and below by a gray

border.

Unit tests pass.
Verified the following in Chrome and Firefox:

Expansion links are only visible if the diff fragment can be

   expanded in that direction.
When a review request is loaded, the expansion controls are hidden.
Upon hovering over a diff fragment, the expansion controls expand.
Upon stopping hovering over a diff fragment, this is a delay before

   the expansion controls become hidden again.
Upon stopping hovering over a diff fragment, if the mouse hovers

   over the diff fragment again before the delay expires, the

   expansion controls do not become hidden.
Clicking on the "expand by 20" lines buttons expands the context by

   20 lines in the appropriate direction.
Click on the expand to header button expands to that header.
Expanding multiple times expands upon the current context.
The collapse button smoothly follows scrolling, staying in the

   centre of the screen (if the fragment takes up the whole view) or

   the centre of the fragment (if the fragment does not take up the

   whole view).
Clicking the collapse button makes all context hidden and removes

   the collapse button.
The lines originally flagged for review have a border around them.
When the file is a new file, only one line number column is

   visible.
If upon contracting a diff fragment the mouse is no longer above

   the fragment for a period, the diff fragment's controls will become

   hidden.

Also verified the following:

An invalid lines_of_context query parameter is interpreted as

   0 lines of context above and below
If too many lines_of_context query parameter values are passed

   (e.g., 0,0,0,0), it is interpreted as if only the first two

   values were present.
If the lines_of_context query parameter is passed with no value,

   it is interpreted as 0,0
If the lines_of_context query parameter is passed with only one

   value, it is interpreted as both the above and below context.