Add ability for Super Admins to merge Orgs

[briri]

Fixes #2398.

Changes proposed in this PR:

• Add a 'Merge' tab to the edit Org page that is only visible to Super Admins
• Add routes, controller actions, policies and views to support:
  • A first-pass analysis of what will be merged (e.g. contact_email, plans, users, etc.)
  • The actual merge
• Add helper and presenter to display analysis of merge
• Tests for controller, presenter and feature

The primary use-case here is to merge an Org that was created automatically when a user did not select one from the typeahead. For example if the user typed 'USGS' instead of selecting 'United States Geological Survey (usgs.gov)'. In this scenario the Super Admin would select the 'USGS' org from the main Admin -> Organisations menu, click on the 'Merge' tab and then select 'United States Geological Survey (usgs.gov)' from that typeahead.

The following screenshot shows the 'Merge' tab when the user first clicks on it:

The remaining screenshots show the Analysis results and all messaging:


etc. etc.

[briri]

This will need to be updated in conjunction with any changes made to #2724

[briri]

Just noting that we are testing this out in stage and will be deploying to production next week. The functionality is only available to Super Admins

[briri]

@raycarrick-ed can you merge this one when you have a chance?