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