Server sent events are not sent

[tnotheis]

Readiness checklist

• I added/updated unit tests.
• I added/updated integration tests.
• I ensured that the PR title is good enough for the changelog.
• I labeled the PR.

This PR fixes multiple issues with server sent events:

• First and foremost, it URL-encodes the enmeshed address in the URL. Otherwise the HttpClient thinks that the did: part should represent a URL scheme like http: and fails, because it doesn't support this scheme. See 2ed3e96 for the corresponding commit.
• Due to a recent change to the PushService, it is now required to add ASP.NET Identity to the service collection. See bceae43
• For some reason, I had to replace AddAuthorization with AddAuthorizationBuilder. Before that, a call to the /api/v1/sse route returned a 302 (a redirect to the auth endpoint). See 64d63cc.
• I catch the OperationCancelledException in order to prevent it from being logged (this caused a lot of noise before). See 5e8b2f9.
• I removed the return value from the SseController.Subscribe method. Before it threw an exception in case of a cancelled request, because it tried to return Ok even though there was no response stream to write to (because the request was cancelled). See 3d47ac7.