Expose Places importer

[rnewman]

For mozilla/tofino#514 we need some way to get data from a Firefox profile into Tofino, which means into the UAS.

Pulling data out of Places in the Tofino main process/a frontend process/a command-line tool, then schlepping it over HTTP to the UAS, seems redundant when we have that importing code available to us in ClojureScript.

This issue tracks lifting the Places importer out of Datomish itself, adding it to this repo, and then exposing some mechanism for triggering it with a file path.