Include a link to HMDA

Builds on #80.

Adds a link to the HMDA explorer page as a stop gap while we figure out downloading requirements. The only mismatch we have is that the HMDA explorer is indexed by metro *division* (if present) rather than always by MSA.

Luckily, importing shapes is easy, so this pull request adds the tweaks needed to import metro divisions and uses them when building the HMDA explorer url.