MobileApps: Switch to key_value and spec modules

[d00rman]

This PR brings many changes to the MobileApps endpoints in RESTBase. Firstly, this a disruptive change for the public API:

• /page/mobile-html ceases to exist
• -html is being dropped from route names of the other endpoints

Secondly, we now store only the latest render for each title, and do so only for the mobile-sections endpoint. Two key_value buckets are used for that: mobileapps.lead and mobileapps.remaining, each storing a part of the response, respectively so that they can be served efficiently, being more popular then the main mobile-sections route.

Thirdly, with regards to refreshing the content, only the mobile-sections route causes content to be regenerated; sending a no-cache request to mobile-sections-lead and mobile-sections-remaining will cause RESTBase to simply return the latest stored content.

Finally, all of the paths have been simplified and moved out of the configuration file into its own spec module.

This PR also brings some improvements and fixes. Namely:

• the key_value module now accepts compression and updates settings
• some Content-Type checks in tests have the additional (now-required) charset component
• in the handler templating code, added return_if to the list of components determining if a request is simple or not
• for route loading, use hasOwnProperty() to check if a function for a route/method has been defined

[d00rman]

a bit late, but cc @wikimedia/services