Group join rules to be more intuitive

[nadonomy]

Join rules have grown organically over time, at the time of writing now amounting to 4:

1. Invite only
2. Public
3. Members of a space
4. Ask to join (knocking)

This means users need to rationalise 4 things before they can make a simple, intuitive decision. From a brief internal discussion (h/t @t3chguy @clokep) we should consider instead first grouping into 'Invite only' & 'Public', with advanced modifiers within them, e.g.:

(x) Invite only
     [ ] Allow people to knock
     [ ] Allow people to join if they're part of rooms: [blah, blah ]
( ) Public

When we pick up this enhancement, we should also remember to consider both room creation and room settings.

[nadonomy]

@t3chguy @clokep feel free to expand with any more nuance you think we should capture.

@amshakal @VolkerJunginger heads up - we should factor this thinking into Element X as we go!

[t3chguy]

Maybe address management and room directory publishing live under Public too to group those sensibly