appview,knotserver: show repo license as metadata

This PR introduces licenses as a new piece of metadata associated with a repo. This was modeled to function similarly to how repo language detection already works. On ref updates to the default branch of a repo, run `licensecheck` across files in the root of the directory that match a license-like filename (`LICENSE`, `unlicense.txt`, etc.). If there are any matches with a confidence level of 75% or greater, the best match is stored as the license for this repository. Repos with a populated license entry have a pill with a scale icon shown in the "metadata" header, after the URL entry (if present) and before any labels. Clicking the pill will link to `/org/repo/blob/HEAD/<path>` to navigate to the license file directly.