Allow filtering on Array fields.

[gmaclennan]

This is a little tricky because Mapbox filters do not work on arrays. Instead we flatten the array into:

arrayField.0: arrayValue0, arrayField.1: arrayValue1, arrayField.2: arrayValue2

Then pass the same filter to each of these and group with an ‘any’ filter.

Things not working quite right, or untested:

• Arrays that contain non-string fields
• The Popup component does not render the title or subtitle correctly if these are array fields. Currently the Popup gets its props from the Mapbox component, which has the flattened data, and the Popup is rendered in a separate context without access to the redux store, so we can't access the selectors.
• Some of the code for array values is a little fragile and edge-cases might break things.
• ODK Collect tends to return space-separated fields rather than array fields, so we need to detect and transform those.

[mojodna]

My eye snagged on this. Is this the same as:

v.forEach(w => {
  p[w] = (p[w] || 0) + 1
})

[gmaclennan]

Yes it would be, although I try to avoid this pattern when I remember to, because you can get bugs when you have falsey values (e.g. p[w] = 0), although in this case we would be fine, I think the more explicit code is better.

[mojodna]

LGTM on a cursory reading.