Add confirmation prompt for removing a member from the band

[PresidentNick]

resolves #686

[aaronoppenheimer]

A great thing to implement - thanks. Two things:

1. The translations don't work because of the 'fuzzy' comments in the po file. Looking at the po files, there are a lot of fuzzies in there - but we can't just strip them out without checking to make sure the translations are still right (since the message compiler might have done something weird). I'll add an issue to do this, but for now you can do it on your new strings.
2. Instead of using the htmx-confirm dialog you should use the bootbox one like when trashing or archiving a gig - see here. I'm aware that we also use a javascript file static/js/bootstrap-show-modal. This should be cleaned up! But let's not add a third one.

thanks!