[REF] mass_mailing: convert templates to themes and clean code

* Remove mass mailing template switching system (lemon, airmail, ...)
and replace it with a theme selector which only toggle a different
class on the body.
* Lint JS files.
* Change the way the editor is overridden by mass_mailing in a more
odoo way (put the JS in assets and check on runtime if it must be
applied.

+ fix editor style
+ remove useless code
+ remove odoo template