Feature/calibre content server

[benjitobz]

NOTE: I created a PR in Calibre-Web and Calibre-Web-Automated, the implementation is the same across all three.

Been meaning to do this for awhile. Calibre-Web already knows where the library and the calibre binaries are, but there's no way to run calibre's own content server from it.

What this adds

Calibre-Web runs calibre-server as a child process, configured under Basic
Configuration → Feature Configuration:

• "Enable Calibre Content Server" checkbox, with port/username/password appearing once
  it's ticked — same data-control/data-related pattern as the Kobo and upload settings.
• "Allow Anonymous Writes" hides the credential fields and starts the server with
  --enable-local-write --trusted-ips 0.0.0.0/0,::/0.
• With credentials it starts with --enable-auth --auth-mode basic and a generated user
  db, so ordinary HTTP clients can authenticate.
• Starts on boot when enabled, restarts on save only if one of its own settings changed,
  and stops on shutdown and before the os.execv restart so the new process doesn't hit
  a port conflict.

Port, username and password can also come from CALIBRE_SERVER_PORT,
CALIBRE_SERVER_USERNAME and CALIBRE_SERVER_PASSWORD for docker setups. Env values win,
never get written to the db, and their fields render disabled.

[benjitobz]

Noticed some issues with this, going to close and reopen once I figure out a better approach.