Editable email templates

Targets #1444 (closed)

Summary

1. Create, edit and delete functionality for email templates.
2. Show last edited timestamp for an email template.
3. Translation updates for create/edit template page.
4. Seeder update to set task notification email template type so that it can be checked to not to be deleted. This is a temporary code and should be moved to the logic where we are setting up the email template in the first place.
5. Updated CMS > pages editor layout to take up full window space like the email template page.