Convert the mass Beta Feature into feature-level ones, enabling formulæ

Make every module that was formerly part of the experimental module its own
Beta Feature. For now, all of them are commented out except for formulæ editing
(mwmath). The "experimental" Beta Feature bucket is no more, but the preference
remains so that it can be set to both true and hidden on MediaWiki.org to let
all the different experimental flags.

The feature icons, previous mis-named "logo" and mis-identified as "screenshots"
are now renamed. One for the mwmath Beta Feature has been added (the rest need
creating before those can be enabled). The i18n descriptions now each identify
that the feature is an experimental one, and that caution is needed.

Change-Id: I28862f3e62f5c78aca33f11265aced1db67f4725
Merge "Convert the mass Beta Feature into feature-level ones, enabling formulæ"