Tweak the layout of the experiments code

In order to ease extracting the experiments code - either upstreaming
it to mediawiki/core or creating a micro-library - the core function,
bucket, needs to be made agnostic of MobileFrontend.

• Create the getBucketInternal function from the bucket function and those parts of the getBucket function that aren't MobileFrontend- specific
• Make getBucket wrap the getBucketInternal function, passing in the MobileFrontend experiments configuration, wgMFExperiments, and the user's session ID

Bug: T107592
Change-Id: Id23edeffb3cd025bf0db7f80e4133e5334e704f7