SS-1229 Fix of the bug not saving deleted on date

[akochari]

Description

This PR fixes the issue SS-1229. We didn't save the deleted_on date when an app was deleted by the users. In addition, I added timezone info to the saved value. I also added a unit test to check that this field was populated on deletion.
This PR also fixes the issue SS-1238. Persistent volumes that were deleted were shown as options when launching a file manager or notebooks.
Finally, unrelated change - fixed the links to documentation from the app creation/edit form because we missed that in past PRs.

Checklist

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• This pull request is against develop branch (not applicable for hotfixes)
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[alfredeen]

LGTM. Good work.