[Task] Safe mode - first steps

[Gnuxie]

Main story: #1

Acceptance criteria for this task:

• Safe mode is locked behind a feature flag while we have no diagnostic or config editing commands.
• It is obvious to the moderator that Draupnir is in safe mode, even if days have passed since the failure happened and there is lots of scrollback to reach that point
• It is obvious how to attempt to restart Draupnir normally in any notification relating to Draupnir being in safe mode
• Appservice draupnir also can boot into and from safe mode
• Draupnir has a command to go into safe mode.
• Draupnir can still be started without safe mode, but safe mode is the default. (Use case is people who know what they're doing and want draupnir to keep being restarted by systemd if it fails while they're out)

Details

This should be a distinct bot from draupnir that gets created when draupnir cannot start.

It should have a distinct command table, with one command that restarts the bot and displays help.

Every single command or message should show a warning that draupnir is in safe mode in the management room.

There's something to show the Error or ResultError that caused draupnir to start in safe mode. Possibly a command that can show the cause and details about it.

Config

Config should be something like this:

safeMode:
  enabled: boolean

PRs

• Initial safe mode (without editor or recovery options) Draupnir#565
• appservice provisioned draupnir safe mode test Draupnir#567

[Gnuxie]

Still need to do:

• Exceptions are serialized poorly to Draupnir's log Gnuxie/matrix-protection-suite#62
• change command dispatcher to verify all commands in the provided table have a renderer in the provided interface adaptor Draupnir#566