Fix form bootstrap repeat icons

This PR does the following:

1. Fix a tiny bug-ette in HTML helper adding unnecessary space character. (Sad I know!)
2. Make the +/- Repeat Groups in form Bootstrap templates be styled as bootstrap button groups.
3. Change the + icon to a cleaner plus symbol and the - to an X.
4. Do 3. in the sub-options page in back-end e.g. for radiobuttons plugin.
5. Minor string change to make Title Case.