Added room join form in index page

[MrTip0]

Added a separate area in the index page to avoid editing directly the URL

[jmattheis]

I'd prefer changing the create room button to something like join or create room. Then we don't need two different forms.

[MrTip0]

I edited it, now in the create form, there is the option to join the room if it already exists.

[jmattheis]

I think we don't need another check box for that. We can hardcode joinIfExist: true and change the label of the button to something like "join or create room".