Allow access to public Local Sites by anonymous users.

Local Sites were recently given a flag to control whether they could be
publicly accessed. This still required the user to be logged in, though.

This change allows access to public Local Sites by anonymous users, when
anonymous access is turned on for the installation.

A large part of this change is just cleaning up reviews/views.py. Nearly
all the functions either did their own LocalSite checks or assumed
_find_review_request would do it. We now consolidate the logic in a
decorator.

The new logic works like this:

* If the user is anonymous, and the site is private, they will get
  redirected to the Login page.

* If the user is anonymous, and the site is public, they will be able
  to access the page's view (which still may deny them for other
  reasons, of course).

* If the user is logged in, and the site is private, and they don't
  have access, they will get a Permission Denied error.

* If the user is logged in, and the site is public, they will be able
  to access the page's view.

There's also a better experience for the dashboard. Now, whether using
LocalSites or not, anonymous users will get redirected to the All Review
Requests page instead of the Dashboard by default. Either page will then
be able to do their own checks, showing a Permission Denied or login box
as appropriate, given the settings.
Fix a regression involving the new visible check in the repository page.

The new visible check that was recently added broke RBCommons, because
of the lack of a displayed 'visible' checkbox. Since the new repository
work for Review Board may also be impacted by this in the future, I'm
fixing it here. We now try to get the value, and fall back on it being
visible.
Fix Dashboard to-group for unjoined public groups

This change updates the dashboard to use Group.is_accessible_by instead of
limiting things to users' joined groups. This allows users to use the dashboard
to see watched groups as well as joined groups.

Testing Done: Ran unit tests.

Bugs closed: 3204

Reviewed at https://reviews.reviewboard.org/r/5396/