INFRA-104: create-pot-files-extensions.sh: add env variable CIVI_KEEP_IT_QUIET to reduce the number of messages which are not relevant to the automatic process (we still want some feedback, to have a better jenkins trail of what is happening..)

INFRA-104: civiextensions-update-transifex.php: many cleanups, finish add/push to Transifex of new resources, add/commit to the git repo.

INFRA-104: string extraction for published extension translations

Allow to set the potdir using the POTDIR environment variable.
This makes it easier to pass a specific script argument, but not need
to pass the other arguments which can be auto-detected by this script
using the info.xml of the extension.