Remove some bits of the compare interface if the user isn't signed in.
Notably, they don't need to see the "New Pull Request" button box nor the
hidden form that would fail to submit due to the POST request continuing to
require proper privileges.

Follow-up commits will improve the UI a bit around this, removing some
"Pull Request" verbiage in favor of "Compare."

This change requires pulling in the BaseRepo unconditionally and
recording if the pull request is in-fact not allowed
(.PullRequestCtx.Allowed). If the user isn't allowed to create a pull
request, either because this isn't a fork or same-fork branch PRs aren't
allowed, then we'll name the button "Compare" instead of "Pull Request."

When viewing the branch listing as an unauthenticated user, you'll get
"Pull Request" buttons. use the new "Compare" verbiage instead, which
matches GitHub behavior when you can't issue a pull request from the
branches.