Make Minerva's skin name translatable

Other skins already do this. Many languages just usually translate
everything or need to adjust foreign words, others need to
transliterate the name to different scripts (like Cyryllic).

The 'skinname-minerva' message is automatically used by
Special:Preferences (when the skin is not disabled, like Minerva
currently is).

Updated 'beta-feature-minerva' and 'beta-feature-minerva-description'
messages to refer to existing translations.

Change-Id: Ie0444ed1cb2695f8e4fef0eca792641a4a38a573
Merge "Make Minerva's skin name translatable"