fix(invitefriends): check if registration is allowed before display menu item and pages

Refs #6356
Fixes #6400
chore(invitefriends): form labels

Format labels according to #6356, adds email label and moves instructions into its own div, removes leading spaces in email notifications

Refs #6400
fix(invitefriends): make invite form sticky
chore(invitefriends): use p tag instead of div for instructions
Merge pull request #6401 from hypeJunction/invitefriends

fix(invitefriends): check if registration is allowed before displaying...
Merge pull request #3 from Elgg/master

同步更新