Add custom config.json in global account data

[u9000-Nine]

Feature description

Right now, there is no way to customize various client setting when using the web version of Cinny (which most users do). It would be nice if we could store a custom version of config.json in the global account data which overrides or adds to the settings set in the hosted config.json.

This would bring several improvements:

• Users would be able to customize settings such as the homeserver directory list, and other more involved settings
• We would be able to sync Cinny settings, without any hosting on Cinny's end
  • We could even save more settings to this, which will be especially useful with the upcoming theme PRs

Problems that need to be solved:

• We need to consider what options should be overwritten by the user config.json (if any).
• If adding more settings, such as those in the regular settings menu, we need to consider which should be synced.
• Otherwise, this should actually be fairly easy to implement considering the benefits

[u9000-Nine]

See also: Support default user settings in config.json (e.g., hideMembershipEvents) · Issue #2592 · cinnyapp/cinny (Farside)

[Yashh56]

This feature will be very useful for web users. Is anyone working on it or Can I work on this issue?