Communication: Tagging other user does not work

[julian-wls]

Description

When typing '#', a list with the different channels is presented to let the user select a channel to tag. Lectures and exercises are shown in the list as well. When typing a new word starting with '@', the user expects to see a list similar to the list of channels to pick one of the available members of the current channel. However, no such list is displayed. See the screenshots below for more context.

Suggestion

Line 254 in ConversationViewModel defines tag chars consisting of '@' and '#'.
There seems to be an issue with the function produceUserMentionAutoCompleteHints.

Screenshots

@:

#: