Uploading to a protected title will allow the user to choose a new name instead of...

Uploading to a protected title will allow the user to choose a new name instead of showing an error page
* Made validateNameAndOverwrite protected and moved it to validateName since overwriting is now checked in verifyPermissions()
* Fixed mime verification in case getTitle was not yet called
* Checking for overwrites no longer uses $wgUser

Other changes:
* convertVerifyErrorToStatus now works
* Allow passing the session key to stashSession in UploadFromStash as well