Convert articles on the website (v1.2.0)

## What changed

- **Converter form on rekopub.monoexp.com**, under the summary: paste an
  article address, choose EPUB (the default), KEPUB or Both, and get
  download links. "Links work for 24 hours." is shown above them.
- **`rekopub-web`** (`cmd/rekopub-web`, `internal/web`): a small server on
  127.0.0.1:8004, run as `goapp@rekopub-web` behind nginx. For each
  conversion it runs the `rekopub` command as a separate process, saves the
  books in a new folder with a random name and serves them from
  `/books/<folder>/<file>`. Books are deleted after 24 hours.
- **Limits**, all settable in `/etc/goapps/rekopub-web.env`:
  - 50 conversions in 15 minutes for everyone together. Over that, the form
    is disabled and says "Conversions are temporarily paused. Please try
    again in N minutes."
  - 10 per visitor in 15 minutes, so one visitor can't use up everyone's.
  - 2 conversions running at once, up to 10 waiting (a minute at most).
  - 2 GB of kept books; over that, conversions pause until old books are
    deleted.
  - nginx adds a per-address rate limit in front (6 conversion requests a
    minute, burst 5).
- **New CLI flag `-NOLOCAL`**: refuses local and private addresses even when
  `-URL` is one. Without it, `rekopub -URL http://localtest.me:3000/` turns
  the server's own Gogs page into a book. The website always uses it, and
  also checks the address itself before running anything.
- **"Download / Use ReKoPub"** section title above Download and Usage, which
  become its subsections.
- `deploy/`: the nginx site file and an example settings file.
- `VERSION` 1.2.0.

## Notes

- Needs bajdev/devops PR "cli kind: also deploy a server" merged and
  installed first: it builds `./cmd/rekopub-web` and a server `rekopub`
  into `/opt/apps/rekopub-web/` and restarts `goapp@rekopub-web` before
  publishing the site.
- Server setup before the merge (see the PR description for who does what):
  `/opt/apps/rekopub-web/data/`, `/etc/goapps/rekopub-web.env` with
  `PORT=8004`, the new nginx site file.
- The form works without JavaScript too: the server then answers with a
  page of its own.
- Cross-site form posts are refused, so other sites can't use the converter
  through their visitors' browsers.

## Testing

- `go vet ./...`, `go test ./...` and `go test -race ./internal/web`.
- New tests: `-NOLOCAL` (with the real pipeline against a local server),
  and the server: form checks, local addresses, both limits, busy, storage
  limit, failed conversions, unexpected output paths, link expiry and
  path tricks on `/books/`, deleting old books, HTML answers (escaped),
  cross-site posts, visitor keys for IPv4/IPv6, and the real process
  runner with a stand-in script.
- Ran the whole stack locally: the built `rekopub` and `rekopub-web`
  behind nginx with this PR's server block (without TLS). Converted real
  pages to EPUB, KEPUB and both, downloaded them, and checked: local
  addresses refused (`localtest.me`, `127.0.0.1`, `[::1]`), the per-visitor
  limit, nginx's 429, five conversions sent at once (two at a time, each in
  its own folder), and the page in its normal, finished and paused states.