Remove additional /v1 when computing internalRestbaseURL from
virtualRestConfig as this will be added in getUrl in MathRestbaseInterface.
internalRestbaseURL was introduced in I2fbc441955a1fe417f264f31f3729ce0715f7f16.
Bug: T334842
Bug: T338381
Change-Id: If2719fb9106e1414ce8814a2fbafa5dcedb84e39
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
