This commit adds a `jitsi_server_url` field to the Realm model, which
will be used to save the URL of the custom Jitsi Meet server. In
the database, `None` will encode the server-level default. We can't
readily use `None` in the API, as it could be confused with "field not
sent". Therefore, we will use the string "default" for this purpose.
We have also introduced `server_jitsi_server_url` in the `/register`
API. This will be used to display the server's default Jitsi server
URL in the settings UI.
The existing `jitsi_server_url` will now be calculated as
`realm_jitsi_server_url || server_jitsi_server_url`.
Fixes a part of #17914.
Co-authored-by: Gaurav Pandey <gauravguitarrocks@gmail.com>
Hemant Umre