Self-checks against the .well-known URIs look for the HTTP header
"Access-Control-Allow-Origin" indicating that the remode endpoint
supports CORS. But the remote server is not required to include
said header in the response if the HTTP request does not include
the "Origin" header. This is in accordance with the specification
[1] stating: 'A CORS request is an HTTP request that includes an
"Origin" header.'

This is in fact true for Gitlab pages hosting and that's why the
issue was identified.

Let's specify "Origin" header in the respective uri tasks performing
the HTTP request and ensure a CORS request.

[1] https://fetch.spec.whatwg.org/#http-requests

By default, `--tags=self-check` no longer validates certificates
when `matrix_ssl_retrieval_method` is set to `self-signed`.

Besides this default, people can also enable/disable validation using the
individual role variables manually.

Fixes #124 (Github Issue)

The Server spec says that redirects should be followed for
`/.well-known/matrix/server`. So we follow them.

The Client-Server specs doesn't mention redirects, so we don't
follow redirects there.