Store Integration Manager preferences in account data and allow user to change them somewhere sensible

[lampholder]

The User Settings UX for #4913, basically.

[lampholder]

I don't think there's any benefit to my mocking up a UX for this - there are quite a few places one could put this UX :\

It could be implemented in User Settings (along the lines of #10094) or it could be in the IM UX somewhere).

What we need (as a minimum) is for a user to be able to:

• choose not to use an IM at all
  • and Riot should handle this gracefully
  • they should be made aware that this isn't going to do anything to existing widgets
• choose a different IM
  • we should verify that it is an IM
  • we should prompt the user to ready and accept the IM's policy docs
    • (does this need to be done at config change, or at first use?)
  • existing widgets should 'just work'

[turt2live]

proposal for this: matrix-org/matrix-spec-proposals#1957 (cc from #4913)

[nadonomy]

After validating today, latest comps are in Zeplin, as a part of registration & settings: https://zpl.io/brMdWo3