This new MathInternalRestbaseURL setting allows the internal URL of
Mathoid to be configured directly, instead of relying on
VirtualRestConfig.
This patch moves the fallback logic for the internal and external API
URL into an extension registration callback. This way, we can keep the
application logic simple, and we can issue config warnings that will
cause the updater to fail, forcing wiki owners to update their config.
Bug: T334842
Change-Id: I2fbc441955a1fe417f264f31f3729ce0715f7f16
