Favourite layers

[RobQuincey-DC]

Allow logged in (and potentially anonymous users) to mark layers as 'Favourites', which will put them into a 'My Favourites' (or similar name) folder that is pinned to the top of the layer control.

Logged in users favourites will be persisted in the database. Anonymous users (if we decide they can have it) would be persisted in localStorage.

Only real question is whether favourites should be per version or shared across versions.