Add form in admin site for AutomaticRunGroups.

AutomaticRunGroups can now be created, modified, or deleted through the admin
site by specifying a name, file regex, profile(s), local site (optional), and
repository (optional). See the attached screenshots.

Testing Done:
- Tried adding AutomaticRunGroups with invalid fields (a missing required
  field (name, file regex, profile), an invalid file regex, and a local site
  that does not exist), and saw the appropriate errors on the form.
- Added AutomaticRunGroups with valid fields, and saw them listed in the admin
  panel and in the database.
- Displayed the list of AutomaticRunGroups.
- Modified an AutomaticRunGroup and saved the changes.
- Deleted AutomaticRunGroups, and saw them removed.

Reviewed at https://reviews.reviewboard.org/r/6221/
Automatically execute tools based on AutomaticRunGroups.

Previously, all tools with `run_automatically` set to True were automatically
executed on every published review request. This has been broken since the
addition of tool profiles.

This change fixes automatic execution of tools and changes it such that when a
'review request published' signal is received, tools are automatically executed
only if a set of rules (specified as AutomaticRunGroups) are followed. An
AutomaticRunGroup is very similar to Review Board's DefaultReviewer - it is
basically a set of tool profiles which will be automatically executed when a
file in the diff matches the file regex specified.

Testing Done:
Without any AutomaticRunGroups created, I published a review request with a
diff, and did not see a tool execution get created.

I then added some AutomaticRunGroups, and tried several cases where a tool
execution was not created (as expected):
- Published a new review request with no diff.
- Updated a review request's title (diff was not added).
- Published a review request in a repository that did not match any
  AutomaticRunGroup's repos.
- Published a review request with files that did not match any
  AutomaticRunGroup's file regexes.

I also tried publishing review requests where tool executions were created, and
reviews were posted by Review Bot:
- Matched an AutomaticRunGroup with a single profile.
- Matched an AutomaticRunGroup with multiple profiles.
- Matched several AutomaticRunGroups with overlapping sets of profiles, and saw
  that only unique tool profiles were executed.

Reviewed at https://reviews.reviewboard.org/r/6222/
Add form in admin site for ManualPermissions.

ManualPermissions can now be created, modified, or deleted through the admin
site. I decided to go with a raw ID field for `user` since a drop-down isn't
the best for Review Board instances with many users, but we can change this in
the future if we'd like to.

Testing Done:
- Added, modified, and deleted ManualPermissions successfully.
- Tried entering invalid IDs for the user and local site, and got the expected
  error messages.

Reviewed at https://reviews.reviewboard.org/r/6225/