Allow people to hide repositories which no longer exist.

The repository form validation always checked that the repository existed, but
this neglected the case where someone had decomissioned an old repository but
wanted to keep the review comments around.

This change makes it so people can hide repositories even if they no longer
exist.

Testing done:
Took a repository that wasn't accessible (my local p4 without p4d running) and
tried to save it. Saw that it failed. Un-checked "Visible" and saved
sucessfully.

Fixes bug 2361.

Reviewed at https://reviews.reviewboard.org/r/5360/