Change "Konversation Posten" to "Nachricht Posten"

[westei]

The "Konversation Posten" button should be changed to only post the current Message.

The current functionality originates from the Related Conversation search and is no longer valid as results represent a small part of a Conversation and not the conversation as a whole.

Because of that the Button should be renamed to "Nachricht posten" and it should only post this message.

We might bring back the old functionality as part of #236 in a later release