Remove the unused FirefoggChunkedUpload extension.
Remove the unused ResumableUpload extension.
Remove legacy wfGetSessionKey() function.

The wfGetSessionKey() function is only relevant to MemcachedSessions
logic and is called in one place. Moreover, the function returns the
same value regardless of the environment or wiki or whatever.
Introduce dummy Helios code.
Replace hardcoded references to ExternalUser_Wikia with variables.
Merge pull request #5549 from Wikia/release-254-helios-preparations-and-cleanups-for-dec-3

Minor cleanups and refactors as part of the Helios project.