Bug 35079: Rebuild POT files only if necessary or asked explicitely

Bug 35079: Rebuild POT files only if necessary or asked explicitely

By default, gulp tasks po:create and po:update won't rebuild the POT
files if they already exist.
Both tasks gained a new option --force-extract to rebuild them
unconditionally

This makes it possible to create/update PO files for multiple languages
sequentially without rebuilding the POT file for each language.
For instance:

    gulp po:update --lang fr-FR
    # do something with the PO files
    gulp po:update --lang en-GB
    # ...

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>