Make base font repository path configurable via $wgULSFontRepositoryBasePath

This patch adds a configuration variable, '$wgULSFontRepositoryBasePath', which
specifies the base path to the font repository. By default, the variable is set
to its existing value, which is $wgExtensionAssetsPath +
'/UniversalLanguageSelector/data/fontrepo/fonts/'. However, adding a discrete
configuration variable makes it possible to configure ULS to use a custom URL
scheme that is more suitable for large static assets that change infrequently.

Change-Id: Idbe57072c4f3258757eb691e54cab5429cadcfda
Merge "Make base font repository path configurable via $wgULSFontRepositoryBasePath"