Refactor the pre-commit model a bit.

I'm working on making it so "Update Diff" behaves/looks similar to the
pre-commit section of the "New Review Request" page. This change is a
preperatory step: separating the review request creation from the diff
creation. After this change, `RB.PreCommitModel` should be usable when a review
request already exists, by setting the `reviewRequest` attribute.

Testing Done:
Verified that pre-commit review request creation still worked properly with
both single and parent diffs.

Reviewed at https://reviews.reviewboard.org/r/6121/
Fix "No Summary" in dashboard with draft summaries.

The code in the dashboard that put in the italic "No Summary" for review
requests that had blank summaries wasn't ordered quite right, which meant that
if there was a draft with an empty summary present, the summary field would be
empty, but if it was merely a non-public review request, the summary field
would say "No Summary".

This change re-orders things to always override the displayed summary, whether
or not there's a draft.

Testing done:
Saw that my devserver with a bunch of draft/un-published review requests no
longer showed two different things based on the presence of a draft.

Reviewed at https://reviews.reviewboard.org/r/6136/
Merge branch 'release-2.0.x'
Revert "Make code line numbers work as links"

This reverts commit a9d509042f2384b206300867a54813057b8d8ffc.
Fix post-commit review request creation on local sites.

The post-commit UI on the new review request page wasn't passing through the
local site prefix. This change fixes that.

Testing done:
Created a local site, and added a repo to it. Loaded /s/site/r/new/ and created
a post-commit review request.

Reviewed at https://reviews.reviewboard.org/r/6143/
Merge branch 'release-2.0.x'