Remove: setting "no_http_content_downloads" (#12058)

As we now use HTTPS, it is very likely this will work on most systems.
For systems that do have HTTPS blocked, it will fail instantly,
and it will fallback to TCP anyway. That makes this setting no longer
very useful.

Feature: opt-in survey when exiting a game

On first start-up, the game will ask if you want to participate
in our automated survey. You have to opt-in, and can easily opt-out
(via the Options) at any time.

When opt-in, whenever you exit a game, a JSON blob will be send
to the survey server hosted by OpenTTD. This JSON blob contains
information that gives a global picture of the game just played:
- What settings were used
- How many humans vs AIs
- How long the game has been played
- Basic information about the OS / CPU

All this information is kept very generic, so there is no
chance we send private information to our survey server.
Nothing in the JSON blob could identify you as a person; it
mostly tells about the game played. At any time you can see
what the JSON blob includes, by pressing the "Preview Survey
Results" button in-game.

Fix c6c3d0e6: restore string-based settings in network-private settings (#10765)

By accident the SDTC_SSTR got replaced with SDTC_VAR, which breaks
many of the settings in this file.

Fix: no_http_content_downloads and use_relay_service as private settings (#10762)

Basically, we don't need to know those values when people send in
crash reports.

Change: move sensitive information to secrets.cfg and private information to private.cfg

We often ask people for their openttd.cfg, which now includes their
passwords, usernames, etc. It is easy for people to overlook this,
unwillingly sharing information they shouldn't.

By splitting this information over either private.cfg or secrets.cfg,
we make it more obvious they shouldn't be sharing those files, and
hint to what is inside them.