Merge branch '494-implement-backend-for-reblogging-and-blog-comments' into 'master'

Add support for comments and reblogging to Blog Client

Comments and reblogs need to depend on the post they refer to.
Since message dependencies are limited to one group,
the post and also the comments need to be wrapped
when commented on or reblogged to another blog (and group).

For this reason, in addition to comments, two new wrapping message types
are introduced. They retain all data of the original messages and allow
for reconstruction and signature verification.

This MR breaks backwards compatibility with old blog posts.
It removes the content type, title and parent ID from the post.

Furthermore, it includes one commit that replaces the `Message` in `MessageSharedEvent` with a `MessageId`.

Closes #494

See merge request !285