Add customusermenu theme setting so admins can add items to the new user menu

The new user menu doesn't have much in it by default. We need to give 2.8 users a good way to add new items without having to resort to code hacks, or forcing them to wait until fancier plugins exist for this simple task.

A quick method of doing this is to emulate the example of custommenu which has been used for some time.

A. Add a new text field setting called customusermenu, on the theme settings page, directly under custommenu.

B. Use this as the default text, it will also function as an example:

editmyprofile,moodle|/user/edit.php|edit

messages,message|/message/index.php|message

myfiles,moodle|/user/files.php|addfile

mybadges,badges|/badges/mybadges.php|award

People may not want all these, but it's very easy to remove them. I also like to raise awareness of some of these things.

C. Menu code should look for this setting, parse it and add it after "My home" and before "Log out". The code needs to have good error checking, but any line that fails to parse can just be ignored.

D. The whole menu can be cached in session to avoid this being done on every page load.

E. These rules should be documented on https://docs.moodle.org/28/en/Theme_settings