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.

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.