Separate HTML markup rendering from execute method
Add revision toggle to all history-enabled fields

Previously revision toggle was only visible on fields for which history
was enabled AND stored data existed. Now toggle is by default visible if
field has history enabled BUT if no data is available it only shows a
tooltip (title) explaining current situation.

This update also adds versionControlFields property to Template object,
containing information about which fields of said Template have history
enabled.
Add option to specify render method (HTML or JSON)
Define render method (for future reference)