Create form mixin to add CSS classes to form elements
Partial Bootstrap 3 conversion

TODO: more templates
TODO: fix various bugs
Add various files that we're not using yet, but will need in future
Fix breadcrumbs
Forms in settings
Update blog style
Harmonise styles in settings and replace tabs with pills
Tidy up templates (e.g. remove useless tags and unneeded indentation
Make login/register/inbox add+edit forms slightly less wide on bigger screens
HTML preference is now a checkbox
Limit login attempts

Fixes #117