Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1931

`|to_json` already adds the surrounding double-quotes for the string, so
adding them explicitly means we've got a double-quotes problem.

* Implement MSC 1929 Homeserver Admin Contact
Fixes #1612

For details to the proposed (not accepted yet) MSC, see:
https://github.com/matrix-org/matrix-spec-proposals/blob/hs/proposal-admin-contact-1/proposals/1929-admin-contact.md

* Implement feedback from PR #1931

* Implement feedback from PR #1931

* Fix mixed indentation