Leverage MT merge for project versions

I would like to use MT as another source of translations to copy from when performing TM merge or copy trans. I should be able to tell the source of the translations when looking at its history.

Todo:

• filter of MT translation (GWT)
• restrict to certain permission/role
• The new modal for MT merge in version level (all languages or single language)
• option to override fuzzy or not
• option to save as translated or fuzzy
• schema change to textflowtarget for MT + engine
• fix 1 MT engine from Zanata
• MT credential in system properties
• MT translation should be excluded from TM suggestion
• Any modification will clear the MT flag
• Include attribution into the translated file and UI in Zanata
• MT option only available to pot/po, properties and xml file format as part of legal attribution requirement from the backend engine, e.g. Google

UX: https://github.com/zanata/zanata-platform/pull/919