[IMP][website_mail] improved template designer, added support for backgr...

...ound color, font color and bootstrap buttons(inline css) in mail designer + added new snippets + improved snippet categories

 - Task: https://www.odoo.com/web?#id=9439&view_type=form&model=project.task
 - Pad: https://pad.odoo.com/p/r.7aiZsyw4go0bC7De
So that while editing/designing mail once again, user do not have to select theme he selected last time. Added help attribute for this field.