Add Settings UI for `settings.json`

[Too-Zestyy]

Currently, a basic internal settings file schema is available (and actively written), but is not used. Migrate previously hard-coded logic to use variables from the settings file, preferably using get_sendt_settings() and write_settings_to_database().

[Too-Zestyy]

Base implementation of settings added in commits 62885e0, a188394, 12c9fa1 and f88f7b9.