Support multiple geographies

[ajparsons]

Add support for multiple geographies to LiH, initially to set up an extra map for local authorities, but this is also creating a framework to plug the new constituencies into.

More details in the doc.

[This ticket is part of the new constituency data plan]

• import new constituencies
• enable swapping area types on explore page
• display new boundaries on explore page
• update area page to use area type (also update URL to include this) #334
• update explore page filtering to respect selected area type #335
• handle switching area type on explore page when filters/shaders already selected #336
• import and store overlapping old/new constituencies #337
• New / updated import scripts to translate old constituency datasets to new constituencies #360

[alexander-griffen]

Update

Done so far:

• Adding the AreaType model is ready to review (PR) – merged

Next steps:

• Reviewing prep work document, and comments. Updating to reflect these changes
• Implementing changes to DataSet and DataType models to accomodate multiple AreaTypes
• Implement front-end changes as per mockups in Decide how to display new and old constituencies on the site #264

[zarino]

Done!! 🎉