Given a URL pattern of the form `((?P<capture>[a-z]+)/)?`, a call to
reverse with the kwargs `{'capture': 'foo'}` will result in a
`NoReverseMatch` exception. This occurs because the `capture` param gets
hidden behind a positional parameter and Django will expect the
positional parameter intead of the named parameter for the call to
reverse. We now no longer use capturing groups -- we explicltly mark the
outer group as non-capturing (e.g. `(?:(?P<capture>[a-z]+)/)?`, which
works as expected.

We previously were also using capturing groups in the form of
`?(P<capture>(a|b))`. Not only will a non-capturing group work here, the
group isn't needed at all: a expression of the form `(?P<capture>a|b)`
is sufficient. All instances of this have been fixed.

Testing Done:
- Ran unit tests.
- Viewed the diff fragment view for an interdiff.
- Viewed the e-mail preview views in html and text formats.

Reviewed at https://reviews.reviewboard.org/r/10273/