use less to create button CSS

An example of the power of LESS for this kind of work. Previously
the buttons were pages of bootstrap CSS, cut and pasted and tweaked to fit with Moodle. With
LESS you can mostly just declare what you want things in Moodle to look like in terms of Bootstrap and it does the rest.

It's not perfect, the admin buttons need work, but you get the idea, particularly if you compare buttons.less to bootstrap_buttons.css and then go and look at the compiled output, and search for #buttonscell to see the crazy amount of code generated from those short lines.

https://github.com/ds125v/theme_bootstrap/blob/18c68520f52a9e7366ab55b067a188b27c0bc677/style/bootstrap-min.css

(I just noticed from the diff that I forgot to minify the CSS, but that's not important and actually helps to show the impact of this change, so I'll pretend it was intentional)