subcommunities: determine where request creator will see their request

[slint]

There is no clear place where the user creating a subcommunity request can list all their "outgoing" requests. Some ideas:

• Make it so that the community "Requests" page, also shows outgoing requests (i.e. requests where the community is part of request.created_by).
  • This might be somewhat confusing without making sure that the UI clearly differentiates incoming from outgoing requests. The "Requests" page currently also lists e.g. record inclusion and draft review requests. A toggle filter or facet might be the solution, but in any case, we'll need some mockups to verify.
• Change it so that the request.created_by entity is the user performing the request, instead of the requesting community.

To remedy this, we exceptionally send a request creation "receipt" notification to the requesting community's managers (owners + curators). That way at least they have an "email" proof of the request.