Not all issues can be picked when using @ in a document

### Description of the issue
When using @ in a document, a list appears with employees, issues, documents etc.
In this list only 5 issues and documents are shown, which makes it often impossible to add a link to the correct issue.

### Your environment
* 0.6.250
* DuckDuckGo and Chrome (latest)
* Mac OS

### Steps to reproduce
1) Open the Documents menu
2) Create a document
3) Type @ - a list of employees, companies, issues,. .. documents appears

=> This list is limited and not loaded on demand when typing extra characters

### Expected behaviour
A list of issues that is updated / filtered when typing some characters after the @

### Actual behaviour
- The list shown is not complete
- When typing some characters after the @, most of the time, there are no results because only the issues from the initial list seem to show up

Start at the Documents menu by creating a document and reproducing the @ mention list in the editor. Check how results are filtered after typing characters following @; done means the list can find issues and documents beyond the initial five and updates to match the entered text.

Written by the indexing model from the issue text.