Intermediate commit on the "team join" feature, it does:
- display a team/join/request/ page
- send email to each member of the team when button is pressed
- display a team/join/accept/ page
- fixed several improper HTML rendering
- added one-shot messages to inform users that their action has been performed or not
- fixed the ERROR messages to fit with the bootstrap framework
- finished to handle the 'join team' feature (need to be polished a bit more)