Highlight mentions that concern the user #1924

Follow-up to #1910.

Logic can be adapted from https://github.com/nextcloud/spreed/blob/main/src/components/MessagesList/MessagesGroup/Message/MessagePart/Mention.vue:

1. Mention is for the call or the user
2. Mentions is for a group the user is in (need to load the user groups for that)
3. Mention is for a guest and the user is the guest (no guest support so far, so it can not be implemented)