One frequent request is to have the ability to show tabstops as

something other than 8 characters. When we first started getting this

request, the tab-size CSS rule was very poorly supported, and so there

wasn't much we could do. Many years later, it's been properly

implemented in all the browsers we care about, so it's well past time to

support this use case.
This change lays the framework for doing this, and implements the bare

minimum case of setting a global default for the tabstop size. This

involves several pieces:

A new extra dict has been added to the diff file context and plumbed

  through the diff viewer, views, and JS model.
A new setting for the default has been added to siteconfig and the

DiffSettings object. The settings has been plumbed through a few

  more places in order to get it where it needs to go for this.
Plumb the diff settings/tab size through into the chunk and opcode

  generators, so that they will take it into account (for things like

  indentation change markers).
The DiffReviewableView will check if there's a tab_size key in the

extra object, and use that to set the CSS for the file.

A natural future enhancement to this is to add an extension hook that

can modify the DiffSettings for an individual file, which will allow us

to build an extension that could do things like use .editorconfig files

in order to manage this on a per-path or per-file-type basis. It might

also be nice to give the user some kind of UI for changing the displayed

tabstop width for a file on the client side.
While in here, this fixes up a typo in the diff settings form, and the

diff settings form documentation.

Set the tabstop setting to various options (empty, 0, 4, 8, 12) and

  saw that it was properly reflected in the diff viewer for a change

  involving tabs.
Tested that indentation change markers worked correctly with different

  tabstop settings, including 1 and 2 characters (where the marker is

  truncated to fit in the space).
Ran unit tests.