Add bookmarking feature in diff viewer so users can create and quickly jump

between bookmarks. Intended to help users moving back and forth between

different parts in the diff viewer when doing code reviews.
A "ghost" bookmark will follow the cursor on the right margin of the diff

viewer, which a user can click on to create new bookmarks on the hovered line

of the diff file. Clicking on the created bookmark will scroll the window

to where the bookmarked line is. Bookmarks are saved per diff using browser

Cookie, and will persist for one week.
Usability features include assigning different colors to bookmarks for easy

differentiation, positioning bookmarks on where the target line is relative

to the whole page, and animating the position at bookmark creation so users

can see where the newly created bookmark will reside.

Automated JS test cases:
Ghost bookmark

 - should move along with the cursor on the diff

 - becomes invisible when the cursor is not on diff

 - expands when the cursor approaches

 - shrinks when the cursor moves away

 - creates a new bookmark when clicked on it

 - no ghost bookmark when there already is a bookmark on the hovered line
Bookmarks

 - appears in front of ghost bookmark

 - expands when the cursor approaches

 - shrinks when the cursor moves away

 - will animate on creation

 - on file header should only display the filename

 - on a line in a file should display the filename and the line number

 - on an expanded chunk will hide when the chunk is collapsed

 - on a collapsed chunk will appear when the chunk is expanded

 - should have different colors (for first 10 bookmarks)

 - will be removed when clicked on [x]

 - will scroll the page when clicked

 - can be created from cookies

 - will trigger cookie save event on creation or removal

Following features are tested manually on local devserver:
Bookmarks

 - on an empty line should display nicely

 - on a line that wraps on browser as multiple lines should display nicely

 - added bookmark will appear again after refreshing on the page

 - removed bookmarks will not appear again after refreshing on the page

 - should be positioned where the main bookmark is relative to the whole page

 - are repositioned when window is resized

 - are repositioned when a chunk is expanded/collapsed

 - are repositioned when a file is loaded