Architecture variant: no topics, RFC 8030 for Proprietary Push

[rfc2822]

Variant of the architectural overview, when we drop the requirement for topic support. This would mean

• simplified architecture, especially for on WebDAV server side (no requirement to support proprietary protocols here)
• more load and traffic on both the WebDAV-Push Module and the Redirect Proxy (N requests instead of 1 request per collection update)
• less complexity in Redirect Proxy when it doesn't have to manage subscriptions

[rfc2822]

Closing because this is not our current favorite architecture. Please see #21 for our current favorite (and reasoning) and comment there if you have feedback, which is very welcome.