[ADD] Website KeyCDN integration guide
[IMP] stock_account: stock journal in multicompany

Restrict the property_stock_journal to the main company only. This avoids errors in multicompany when the stock journal of the main company (getting an error company mismatch) is used instead of a more useful message explaining the missing stock journal.
Fixes #3251
[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