Provide option to record untranslated strings in database

This is an old issue, but I wanted to extract translatable strings from the codebase and save them to locales_source, so here we are. Here's what the MR does:

1. Adds a new database table, potx_locales_filesystem_location, you can join with locales_source, that saves the locale id, the module / theme name, the filepath within the module/theme, and the filepath to the module/theme.
2. Adds a function, _potx_write_to_locales_source(), that adds strings to locales_source if they aren't there already, and updates the table mentioned above. This function is called from _potx_build_files().
3. Adds support for TMGMT, a source plugin viewable at /admin/tmgmt/sources/potx_locale/default, where you can see the extracted strings if they have info in potx_locales_filesystem_location, and filter by path and name.
4. It doesn't work for drush. It only works if you use the UI to extract strings /admin/config/regional/translate/extract. That probably should be fixed.