Allow enabling audit mode from advanced settings in addition to the command line parameter

[Zulgrib]

Feature description

• Add a checkbox to enable/disable auditing in the advanced settings. This setting should be ignored if Syncthing's start command have "--audit", in this case it is always enabled.
• Add a string field to set an optional non default path to store the audit. This setting should be ignored if the path is set from Syncthing's start command.

Problem or use case

Syncthing may be supplied in packages where it is not possible to change the start command, preventing users to enable auditing.

Alternatives or workarounds

None