Suggest contributing users first when starting a mention

### Describe the problem

As a maintainer of translations, I sometimes want to ask contributors about their contribution. That can be done through the Comments tab. At the moment when I start typing a `@` symbol for the mention, no suggestions appear. I Suggestions appear when I start typing, but that seems to search through the whole list of known Weblate users. This means that when I'm on the History tab and see a user I want to discuss with, I have to take note of the entire name, because I can't view it once I moved over to the Comments tab to start to create a comment with a mention.

### Solution brainstorm

I'd like to have suggestions based on my project, the language and the string. There are about 50 people that contributed to the whole translation project, 5 that contributed to that language and maybe less that worked on that string. It would make sense to highlight those users first.

This is also the behavior I've become accustomed to from other collaborative software like git hosting and chat applications.

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

_No response_

Start from the Comments tab mention suggestions and compare them with the contributor names shown in the History tab. Trace how the current suggestions are searched, then verify that contributors from the string, language, and project are prioritized while general user search remains available. Done means a maintainer can find relevant contributors quickly when typing @.

Written by the indexing model from the issue text.