[IMP][website_mail] improved template designer, added support for background color, font color and bootstrap buttons(inline css) in mail designer,removed other snippet, changed snippet according to new format,added theme selector in mail_designer, lodeded snippets dynamically based on theme, added font-awesome support in mail snippets, added one controller which convert font-awesome to icon

[IMP] Fixed bug of dropzone,improved behavior of edit mode

[IMP] Improved Behavior for selecting theme while designing a fresh template and some other changes