pg-to-git: retrieve tags from git for releases page.

[russmack]

Hey @steveklabnik , more a loop-in than a PR, I forget when we last spoke.

Replaces the postgres populate with git walking. A cache to avoid repeated walking of git. An endpoint to reload the cache, localhost requests only. Gotham for web, templates currently lazystatic. Removes tools which previously worked with populated postgres, but not git. Adds a project-manager tool in anticipation of providing for multiple projects. Some work towards providing for multiple projects. Updates most dependencies. Replaces deprecated parts of slog.