Collisions: select by Highway Authority

[mvl22]

Request from user (dated 14/Sep/2022) to be able to select data by Highway Authority.

The data has local_authority_highway so an API filter can be enabled in the UI. These use the ONS codings, e.g. E06000001, and those are defined in the codings list.

However, this may not be useful until the download allows a greater number of points in the CSV anyway.