Invite module UI and strings update

Created by: phellmes

I did 2 things in this first attempt to contribute to the code:

I updated the UI template for the "invite" module to reflect the overall styling.

I removed trailing spaces from 2 translatable strings and separated them into additional strings behind them. Trailing spaces tend to be ignored when people translate and don't recognize them (this actually happened with the German translation for the invite message - that's how I found the the problem) so that texts or elements are smashed together later on. I think it's better not to use them. I guess these strings have to be translated again in all languages. But I'm not aware of any other method to solve this.