and turn them into a regular string.

Also removes support for anonymous forum posts.
Closes #698

Also fixed a couple of bugs.

This way the forum and private group client do not need to keep track of
message timestamps themselves and do not need to interact with
post/message factories.

Fixes #350

This is also lays the groundwork for #384

in group metadata to be able to speed up group listings.

Closes #584, #586, #585

Like other recursive operations on the dependency graph, this is
not done in a single transaction to prevent an attacker from creating
arbitrary large transactions.

So at startup, the `ValidationManager` finds and resumes any
unfinished operations, by looking for shared messages with unshared
dependencies.