Add filter by sender/receiver name in inbox (also adds private chat)

[elishamint]

Is your proposal related to a problem?

Private messages and replies list (possibly also mentions) can get very long and specific messages could get hard to find after some time. Messages and replies from unrelated contexts appear mixed up when shown sequentially.

Describe the solution you'd like.

Have the UI provide a seletion box with all sender names (for received messages) and receiver names (for sent messages) found in the list, which on selection will cause only messages of the selected person to be shown.

Describe alternatives you've considered.

It could also work with a button next to the sender name in each message, although I would consider this to be an extra, as the select box is more quickly and intuitively accessible.

Additional context

Having this could enable us to turn the PM system into a private chat. Turn on "view all messages" and select your chat partner. It would even already have editable messages and multiple reply functionality.

[elishamint]

Similar to #376