Implementation of ability to mention/reference entities in messages

[JonasMuehlmann]

Concept:

• When sending messages, the sender can mention/reference the following entities, if they are part of the team:
  • users
  • roles
  • the whole team(@ALL)
  • other messages
  • channels
• The mentions can be included in the message text and will be highlighted (Bold and colored?)
  • When writing to the DB though, the mentionId should be inserted into the message(e.g. Refer to @12345 for more info) since this is robust against renaming mentioned entities
  • The mentionId will be converted into the mentioned entities name(e.g. the channel name MyExampleChannel) when loading messages
• While typing, target entities can be suggested similarly to the user search
  • Messages will probably have to be mentioned by their number, hence the beginning of it's content should be shown in the preview and sent messages in the ui should show their number for easy referencing
• When starting to write a message, all available entities could be loaded asynchronously for quick lookup
• When selecting a mention target, users can filter the search by providing a special character followed by a semicolon(e.g. @u:john to search for users with john in their name, @r:dev to search for dev in the teams roles)

Backend tasks:

• Adjust db to include mentions
• Add MentionService
• Write helpers to resolve entity names from the mentionId
• Write method to list all referencable entities
• Write tests

Frontend tasks:

• Show message's ids
• Suggest entities while typing, and complete and validate them
• Highlight mentions

CREATE TABLE Mentions(
	Id INT IDENTITY(1,1) PRIMARY KEY,
	TargetType NVARCHAR(30) NOT NULL,
	TargetId NVARCHAR(64) NOT NULL,
);