Add bootstrap initialization for l10n update in new branches

Running mw-update-l10n in a new branch that doesn't have an existing
ExtensionsMessages-1.XwmfY config file and/or en l10n cache file can
fail due to PHP environment initialization needs. The missing
ExtensionsMessages is temporarily faked using an empty file. A stub l10n
cache is generated using the rebuildLocalisationCache maintenance
script. Both files are replaced with complete versions by subsequent
processing steps.

Bug: 51174
Change-Id: I5467ac860c828b5967e298bca0a8097fc9a36e6f