config: Add new ConfigRepository

config: Add new ConfigRepository

This, for now, has the focus for holding metadata of each
configuration option inside of MediaWiki (both extensions and core).
It's very close to the Config interface and friends, and uses these
implementations to retrieve the actual value of the
configuration option.

The goal with this change is to implement a basic architecture to
allow to display the values of the curent configuration of the MediaWiki
installation on-wiki (e.g. on a special page). It also provides a
central point, where the currently known options can be fetched
from.

The long-term goal, of course, would be to get a web interface to really
configure the MediaWiki installation. But, this is more like a dream,
then a plan (from the point of view of this commit).

Next steps would be:
 * Enable ExtensionRegistry to "register" the configuration options of
   extensions, so they're available in the repo (done).
 * Find a good way to get mediawiki/core configurations into this repo
 * Work out an overall architecture to display the different possible
   values. (I think about something like different formatters for types).

Change-Id: I9419508eaa85ffc55520db7f33b3e9530fc99f00