Fix word wrapping in review request details fields.

Long strings in the review request details fields (on the right-hand side of
the fields) would increase the width of the entire table, eventually
overlapping the main fields in a very ugly way. This change adds a max-width to
the field spans using javascript to compute the correct width, and specifies a
word-wrap to force browsers to insert line breaks in the middle of words in the
case that it can't break at a natural place.

I've also made our layout recomputation method be throttled to 100ms, which
dramatically improves performance when resizing the window.

Testing done:
* Set a branch that was just one long unbroken string of characters and saw it
  wrap at the width of the details pane.
* Checked that a bunch of comma-separated bug numbers still wrapped at an ideal
  place in the text.
* Changed a label in a field to be longer and saw that the span max-width fit
  everything on the screen.
* Resized the window and saw that things worked correctly.

Fixes bug 3371.

Reviewed at https://reviews.reviewboard.org/r/5951/
Fix error handling in "Upload Diff" and "Add File" dialogs.

These dialogs are still driven by some pretty ancient javascript code in
common.js. All the backend code had been changed to use the new resource
models, but the error handling callback hadn't been updated for the new
attributes. The result of this was that instead of seeing an error in the form,
users would just see an endless "Loading..." spinner.

Testing done:
- Tried to upload a .png file instead of a diff and saw the error shown next to
  the 'diff' field.
- Tried to submit the 'Add File' form without selecting a file and saw the
  error shown next to the 'path' field.

Fixes bug 3413.

Reviewed at https://reviews.reviewboard.org/r/6112/
Merge branch 'release-2.0.x'