Use class/method for each ‘page script.’
Add controller for user list pages.

Fixes #540

Also add canonical tag to question lists and tag pages.

We now use qa-global.js for all the in-page JS (except for qa-admin.js
which regular visitors don’t need).

Clean up some of the JS code

qa-include/app/page.php can now be require()’d without side effects.
Closes #213.