fix(1282): User mention list click issue

Targets #1282 (closed)

Bug description

When typing some characters and then clicking on a user name from the mention list, their name appears in a weird way in the chat input.

Steps to replicate:

1. Go to Kotahi dev and open any discussion where you can tag someone. Type in first few characters so that their name comes up in the suggestion list.
2. Click on the name from the list and it will give something like this below

Summary

1. Updating picker to set the new range based on the characters typed as the suggestion list is filtered (by typing characters). This range indicates the characters that the user has typed already when filtering.