Issue: #326 - Immediate Registration

- Registration Mode: Added the ability to select the registration mode
upon installing SMF to provide further control from unwanted
registrations before the setting can be changed from the admin panel or
for people who are unaware it exists. The setting defaults to Immediate
Registration (registration_mode = 0).

- Cleaned up the syntax of the forum_settings template, removed invalid
HTML5.

Reference: Issue - #326 by @redonesmf

Signed-Off: Matthew "Labradoodle-360" Kerle |
<lab360@simplemachines.org>