Add `highlightMarker` Matrix field to `tours` -> `poi` field layout

[ericdrosas87]

To provide flexibility to the new circle focus feature, we need to provide a new field on the tours -> poi field layout called focusRadius. This should be a dropdown with three options:

1. Large
2. Medium
3. Small

We will match the selection of this field to a map of sizes in the client.

[jeffinnes]

The requirements for this have changed a bit since initially laid out.

A new Matrix field will be added to the Tour POI block to allow for zero-to-many highlight markers to be set. For each highlight marker the user will provide:

• RA
• Dec
• Marker diameter as a decimal factor between 0 - 10