add mob member by url, to reduce friction point, and make it easier to mob with others

[alterisian]

So That: I can easily get involved with the current mob As A: Mob Organiser I Want To: Send a url to a potential collaborator, that allows that to fill in a text field with their name, adding them to the mob.

--

Notes: Is this what is wanted? Discussion: https://twitter.com/alterisian/status/1623294428109582336