Add support for maintenance mode

[latenssi]

• Add an HTTP endpoint to get and set system settings; settings that should be shared with all instances and not requiring a instance restart to update
• Add system setting for maintenance mode
• Add API docs for system settings

When the system is in maintenance mode, all jobs will be halted. Basically this means all Flow transactions are halted until maintenance mode is disabled. Synchronous calls to the API (the resulting transactions) will still go though to Flow, please avoid using them.

Chain listener is also halted in maintenance mode, so no Flow events will be processed during maintenance.

resolves #93

[nanuuki]

There don't seem to be any tests for the new /system endpoints, maybe add some basic test cases for them? :)

[nanuuki]

👍