Fixes #635

Fixes #634

by making it only selectable in detail view

This also adds comments to the blog post detail screen that were
previously missing.

Closes #593

Closes #616

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.

Closes #311

Fixes #539, #564 and #568.

Also properly handle back navigation when trying to add contacts.