Restrict @mention to a task when restrictions are applied

Steps to repoduce

1. Create a test page and add edit restriction only to userA
2. In the same page assign tasks to userB who has no edit permission to the page
3. Try to complete the task
4. userB can't complete and message is shown:
5. Check userB profile >> Tasks
6. userB can't complete and same message is shown.

Use case for this report

These tasks will remain indefinitely, unless restriction are given, task removed or assignee changed, in the user inlinetasks page as the userB (mentioned user for the task) can't edit them.

Proposal

When restrictions are applied, we shouldn't be able to @mention an user for a task that he has no edit restriction as he will not be able to interact with it.

Workaround

1. Ask an user to remove/change the assignee
2. Ask an user remove the task
3. Give restriction ability to userB