Implement MSC 1929 Homeserver Admin Contact

[teutat3s]

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

[teutat3s]

Idea from moanos in the matrix room for vars.yml format:

matrix_homeserver_admin_contacts:
  - moanos:
    matrix_id: @moanos:hyteck.de
    email: ...

[teutat3s]

Thank you for the constructive feedback @spantaleev - I tried to address your suggestions in the latest commit.

[spantaleev]

Thanks! 👍

I've merged it, but had to do some post-merge fixes:

• the YAML syntax for matrix_homeserver_admin_contacts did not work for me. See 3f7498d
• there was a double-quoting issue in /.well-known/matrix/support. See c9e3070

---

I've also added a variable (matrix_well_known_matrix_support_enabled) that controls whether the /.well-known/matrix/support file gets generated or not. This is inspired by the matrix_well_known_matrix_server_enabled variable we have, which does the same for the /.well-known/matrix/server file.

Because matrix-org/matrix-spec-proposals#1929 is not yet accepted, I've defaulted the new matrix_well_known_matrix_support_enabled variable to false, so that we won't generate the /.well-known/matrix/support file by default. We shouldn't be contaiminating people's servers with files that pertain to unaccepted experimental features. The example in docs/configuring-well-known.md has been updated to mention matrix_well_known_matrix_support_enabled.