Reorganize export/share UI

This is a followup to the [map-ui](https://github.com/openstreetmap/openstreetmap-website/pull/328) changes, making better use of the new "Share" sidebar. It moves the more consumer-oriented options -- "Embeddable HTML" and "Image" -- alongside the "Link" option for sharing. The OSM XML "Export" option remains, but no longer needs the prominence of a top bar tab, so is moved to the sidebar alongside GPS Traces under a new "Data" section.

Two additional enhancements are included:

* The oft-requested ability to include a marker in the permalink.
* A shortlist of sources for bulk data downloads is provided when the selected area is too large for export.