Make the login page into a renderer

Currently the formatting for the login page is contained in a PHP file:

https://github.com/ds125v/moodle/blob/master/login/index_form.html

This should be transferred into a renderer class and broken down into individual functions so that themes can control the layout.

This has various potential benefits e.g. using a responsive grid so that the two columns collapse into a single column on small screens. (This is currently feature is partially emulated by the Bootstrapbase theme: https://github.com/ds125v/moodle/blob/master/theme/bootstrapbase/less/moodle/responsive.less#L40-L83 but a more straightforward approach would be better and allow for more possibilities).

The login page will often be one of the first things that a Moodle user sees and so is a prime target for branding and customisation.