Epic: Change neighbourhoods to work on multiple scales

[kimadactyl]

Is your feature request related to a problem? Please describe.
Currently neighbourhoods are all UK wards, the lowest level of political geographical area. This is far too fine a level to do permissions for the wider rollouts over GM and London we have planned.

Describe the solution you'd like
Change the Neighbourhood model and associated join models to allow for flexible assignment of political geographical areas to other entities.

Describe alternatives you've considered
Hard coded models for each area. Inflexible and creates a huge amount of join tables.

Steps to take

In order!

• Convert existing neighbourhood table to new structure #791
• Add all other UK geography data #794
• Add new tests and fix logic for admins with different levels of area access #792
• Improve admin by adding data tables #821
• Update User <-> Neighbourhood picker text to be something like "#{unit_code_value}: #{unit_name}"
• Redo edit neighbourhood view #868
• Add edit link back into neighbourhood admin table table (was in actions column) #861

Additional context

Rough sketch of what the new database model will allow below:

[kimadactyl]

@katjam can you review pls? Made a fresh ticket too. I'm a bit too zoomed into this to write good acceptance criteria!

[kimadactyl]

Updated ticket to have datatables as solution instead of fancy picker

[kimadactyl]

Done!