appview,knotserver: show repo license as metadata
This commit introduces licenses as a new piece of metadata associated
with a repo, like languages. 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`, 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.
The license pill on the repo page links out to the license file.
Signed-off-by: Kevin Yap <me@kevinyap.ca>