Welcome page link to riot bot should be pulled from config.json rather than hard coded

[lampholder]

We should consider the button in the bottom left pulling from config.json rather than being hardcoded (but this is not entirely straightforward - the button says 'Chat with Riot Bot' but we only put the mxid in config.json).

[jryans]

This is quite similar to #9263, though the other issue is about disabling the button.

[lampholder]

Oh yeah, this is basically a dup of #9263; closing