feature: per-server plugin settings

[lukepolo]

A community server can override a plugin's cvars and config file for itself, layered over the plugin page's settings.

• server_plugin_configs table, saved through setServerSettings (plugin_configs), one restart with the rest of the server's settings
• On a community server the server's cvars run after the plugin page's and the mode's, and its config file is laid over the others
• Needs hasura metadata apply for the new table, relationship and action input