No Back Button on Join Room Page

[thigg]

Describe the bug
When selecting Join Room with a mobile layout, there's no way to cancel it and go back to the room list from the UI. The only way to go back is the browser's back button

To Reproduce
Click + on the contacts (main) page, select join room.

Expected behavior
It is possible to leave that page without using browser controls, e.g. when using this page in a webview without browser controls, like in the SailfishOS mobile app that wraps hydrogen: https://openrepos.net/content/thigg/hydrogen

Smartphone (please complete the following information):

• Device: [e.g. iPhone6] Sony Xperia 10 II
• OS: [e.g. iOS8.1] SailfishOS 4.4.0.72
• Browser [e.g. stock browser, safari] stock browser based on gecko (Firefox 72)
• Version [e.g. 22]

Additional context
Related: #903 , #904