There are many services within the `MobileContext` god object that will be extracted on a case-by-case basis. As there are many clients of the `MobileContext` class, it will have to act as a [proxy](https://en.wikipedia.org/wiki/Proxy_pattern) for these new services until those clients are migrated. Since these new dependencies will have to be injected into `MobileContext`, we'll need to create the `MobileFrontend.MobileContext` service.
Note well that while `MobileContext` is a singleton, [its singleton instance can be overridden via `MobileContext::setInstance`](https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/master/includes/MobileContext.php#L121-L128).
[x] Audit `MobileContext::setInstance` class, documenting:
* where and why the call is made
* how the call might be removed
[x] Create the `MobileFrontend.Context` service
[x] For backwards compatibility, make `MobileContext::singleton` defer to `MediaWikiServices`
[x] Remove calls to `MobileContext::setInstance` that can be safely removed
* Introduce `MediaWikiServices` into the #mobilefrontend codebase (819064d)