The PUT /repositories/<n>/ API supports an archive_name field which

can hide the repository and give it a special name. This was going

through the regular repository form to actually handle the save

operation, which meant that if the repository being archived was

inaccessible for some reason, the operation would fail.
This change makes it so we check if the only thing being done is

archiving the repository, and if so we skip using the form and just

update the model itself.

Ran unit tests.