Implement UX for showing and answering private group invitations
As usual, this MR contains several logically separate commits that could be split out into smaller MRs if desired. It consists of two main parts:
* Showing open invitations in the list of private groups with a snackbar
* Showing invitations and responses in the private conversation
For both parts, the existing code was refactored to allow for a smooth implementation and to leave maintainable code behind.



Closes#707
See merge request !357
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.
Null safety annotations
The @NotNullByDefault annotation marks all fields, methods and parameters in a class or package @NotNull, so Android Studio will warn if values that may be null are used. Please use this annotation for new classes, and specify @Nullable for any fields, methods and parameters that may be null.
Injected fields are initialised to null, so injected classes should use @MethodsNotNullByDefault and @ParametersNotNullByDefault, or specify @Nullable for injected fields.
See merge request !349
Forum controller thread safety and tree safety
This branch solves the concurrent forum issues by code restructure and refactoring.
Closes#556Closes#552
See merge request !262
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.
Add one introduction test for modified response message
This is only the first part of #627, but I am putting it up for review already, since the second part will be very similar and issues found here will likely apply to both.
See merge request !306
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