Add Character Counter to Username/Display Name fields on Create a Group page

[urbansheep]

When user is creating a new group we don't inform them about character limit for username and/or displayname fields.

We do that warning for user/group settings now anyway, so we could help users a bit here:

1. Add this informer to the Create account/group dialog as well. Doesn't look like a big change but definitely would make limit visible without users hitting the wall of “Display name is too long”.

2. Also let's make the field of limited size (same as we do for comments/posts limited at 3K/1.5K characters).

3. I'd also show the character counter after username/displayname fields are 75% full, so that it's not appearing so suddenly.