Resolve "Show popup on waiting screen even when call is ongoing"

What does this MR do?

Also show button to join room when the slot has already started. Also the displayed time until the slot starts is now being displayed correctly.

Screenshots

before

after

Does this MR meet the acceptance criteria?

• I have added tests to validate the new functionality.
• I have updated the documentation accordingly.
• I have added translations for all new client-side text.