Add security section to admin menu

Currently it only allows the admin to enable or disable the HTTPS
enforcement, but in the future it could be expanded to further options.

The HTTPS enforcement only allows the admin to enforce it, if he is
connected via HTTPS. (To prevent admins to enable it without a proper
SSL setup)
Removed uneeded HTML tag
Merge pull request #1150 from owncloud/EnableSSL-AdminMenu

Add security section to admin settings to enable the HTTPS enforcement