bring repo into policy compliance; vendor assets; Gitea CI

Vendor the front-end assets and drop the third-party CDN dependencies
(BootstrapCDN is being sunset): the bootstrap 4.0.0 css/js, jquery
3.2.1 slim, and popper 1.12.9 now live under static/ and are served
from the app, byte-for-byte identical to the previous SRI-pinned files.

Migrate CI from Drone to a Gitea Actions workflow that runs
docker build . on push, with the checkout action pinned by SHA.

Bring the repo up to standard:

- add REPO_POLICIES.md, .editorconfig, .dockerignore, .golangci.yml,
  and a comprehensive root-anchored .gitignore
- rewrite the Makefile with the required test/lint/fmt/fmt-check/check/
  docker/hooks targets (golangci-lint, 30s test timeout, verbose rerun
  on failure, check modifies nothing)
- rewrite the Dockerfile as a hash-pinned multistage build: a lint
  stage (golangci-lint), a glibc build+test stage (the legacy sqlite
  driver needs cgo+glibc), and a debian-slim runtime carrying the
  binary, templates, and static assets
- add real tests for the hn package
- bring the code into golangci-lint (default: all) compliance: fix the
  malformed gorm struct tags, check previously-ignored errors, dispatch
  the zerolog error event, avoid a uint->Duration overflow, split long
  functions, and add doc comments — all behaviour-preserving
- expand the README with the required sections

go.mod

@@ -5,7 +5,6 @@ go 1.14
 require (
 	git.eeqj.de/sneak/goutil v0.0.0-20200330224956-7fad5dc142e5
 	github.com/flosch/pongo2 v0.0.0-20190707114632-bbf5a6c351f4
-	github.com/hako/durafmt v0.0.0-20191009132224-3f39dc1ed9f4
 	github.com/jinzhu/gorm v1.9.12
 	github.com/juju/errors v0.0.0-20200330140219-3fe23663418f // indirect
 	github.com/labstack/echo v3.3.10+incompatible