tautulli: reach plex through the mesh, written before Tautulli starts

Tautulli reached plex at 172.18.0.1, the gateway of a HAL network that goes
away with HAL, and the plan was to retype it by hand in the window. Tautulli
now requires plex-api, and where plex is comes from the binding.

Tautulli keeps the connection only in config.ini, reads it at start and
writes its whole config back on every shutdown; its API cannot set it and
its settings form needs an admin login. So a step after start would be
overwritten the moment the container is recreated. The write is made where
nothing can overwrite it: the linuxserver image's custom-init runs
plex/mesh-plex.py as root before Tautulli starts, and the server restarts on
its binding and credential, so a moved plex or an accepted token lands.

It writes only [PMS] keys, only when they differ, every other line byte for
byte: pms_ip, pms_port, pms_ssl and pms_url from the binding; pms_identifier
from plex's /identity; pms_token only when plex takes it. A minted value -
before the operator accepts the server's X-Plex-Token for this pair - is
never written, while the address still is, so Tautulli's own working token
keeps working at plex's new address.

A failure in custom-init is a log line nobody reads, so a run-once `plex`
step, declared last so it gates nothing (ADR 0136), checks what the mesh can
report: plex takes the credential (else it names the secret accept), Tautulli
holds the bound URL, and Tautulli says it is connected. It writes nothing.

The script is kept as plex/mesh-plex.py and plex/50-mesh-plex; module.json
carries copies, and a test fails when they differ. Tests run the script with
python3 against a fake plex (skipped where there is none) and the step
against fakes; `npm test` builds first.

modules/tautulli/tsconfig.json

@@ -8,5 +8,5 @@
     "skipLibCheck": true,
     "noEmit": true
   },
-  "include": ["client.ts", "index.ts", "tools/index.ts"]
+  "include": ["client.ts", "index.ts", "tools/index.ts", "plex/check.ts", "plex/index.ts"]
 }