Fix issue with username lookup for comments from deleted accounts

[Hufschmidt]

Ignoring the whitespace changes I added to put my commit in line with how the latest commits seem to be formatted, this only adds a fallback lookup to the translated value 'deleted' IFF trying to lookup a username for a comment, but the user does not exist (or doesn't have a login) anymore.

This fixes a PHP whoops exception that stops InteractiveVideo objects from being opened when there are comments from deleted accounts.

To test and reproduce

• Create an InteractiveVideo object
• Create and login with temporary account
• Leave comment with temporary account
• Logout and delete temporary account
• Login with normal account and try to open InteractiveVideo object
  -> Throws exception about failed array key lookup (original) matching line 363

Potential Improvements

• Currently this looks up the 'deleted' phrase from ILIAS common translation as a fallback when no account was found. Maybe this could be added to the plugins localization instead.