Search in full name for mention suggestions

### Describe the problem

Sometimes you don't remember the username but only some part of the full name of someone you want to mention in a comment. The mention suggestion box does not offer this user when you start typing part of the full name.

### Solution brainstorm

Also search the full_name field when providing mention suggestions.

Additionally search any matching part, not just the start of the string.

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

The users full name is shown in history tab and other places, so you don't actually see the username that often.