hideConferenceSubject does not apply to prejoin screen

[shawnchin]

hideConferenceSubject does not hide the subject on the prejoin screen, which makes for an unsightly experience when rooms names are UUIDs. The subject is correctly hidden on main conference screen.

We first noticed this when upgrading to JaaS 4953 (Jitsi Meet 7874), and this is reproducible on current meet.jit.si. For example, https://meet.jit.si/YouShouldNotSeeThis#config.hideConferenceSubject=true would show:

This might be regression of #14499.

[shawnchin]

BTW, for anyone else affected by this issue, a temporary workaround while waiting for the fix is to override the subject using localSubject=" " (that's a single space. Empty string won't work).

Example:
https://meet.jit.si/YouShouldNotSeeThis#config.hideConferenceSubject=true&config.localSubject=%22%20%22