update explore page filtering to respect selected area type

[struan]

• update JS to pass the current area type to the filter end point
• update the filtering to select data for the relevant area type

[struan]

I think as part of this I'm going to have to add something to the dataset to indicate which area types it's available for.

I think this makes sense and seems much less fraught then trying to derive the information elsewhere.

I am thinking that adding:

areas_available = models.ManyToManyField(AreaType)

to DataSet is the best option, anyone have any thoughts on this @zarino @alexander-griffen

It will mean yet another update to the imports to add this, although I guess I can maybe assume if this is null then it means it applies to WMC types.

[struan]

Although doh! the data_type has an area_type on it so maybe I don't need this...

[struan]

Having thought about this a bit more, and written some code I think I do need this as it's going to make things much easier.

[zarino]

This is being / has been addressed as part of #344

[zarino]

@struan I think this can be closed now, as #344 has been merged into #350 / multiple-geometry-staging?