Include city tag groupings

[rstrange-scottlogic]

The ECMWF have suggested that in future they would like to add extra cities to the locations list.

In doing so we would like to keep the original concept of the CNN cities, and keep this list separate from any new cities they decide to include.

To aid with this we would like to update the cities to include tags. Cities would be able to have multiple tags, and users would be able to select which tag they would like to display on the main overview page.
TODO : Does Tag work as a descriptor? Maybe city_label, or city_group?

This story includes:

• Updating the import process to include a tag, so that we can store tags against cities in the database. We need to work out how to go about this.
• Updated the UI to allow users to select the Tag
• Once selected, only cities with that tag are displayed on the summary page.