Decouple 3 stages in separate methods

Separate all different functionalities in different methods,
the first one is to _update_all_from_vcs that pull recent
code from remote repository, the second is _update_templates
which regenerates the .pot file for the project, and lastly
_update_all_against_templates which updates each language .po
file using the regenerated .pot file.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>