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.
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 commit breaks backwards compatibility with old blog posts.
It removes the content type, title and parent ID from the post
message structure.
* Implemented in briar-core as a `ScheduledExecutorService`
that gets started when the app starts
* The briar-api has a `FeedManager` interface
that the UI can use to register and unregister feeds
* In this first iteration, feeds are fetched via HTTP(S), not Tor
Closes#484
