Add localization keys for system messages

[rsek]

e.g. ui.notifications?.warn('Doing some system housecleaning, please wait...'). searching the codebase for ui.notifications will probably turn up most of the strings that need localization.

The best place to start here may actually be FVTT's own en.json file. Any string we can borrow from there is one less string that a translator needs to work on (though we may want to prepend it like foundry-ironsworn: blah blah blah to make it clear where the message is coming from.