Improve text on todo list when the todo action comes from yourself

Improved todo message when an issue is assigned to yourself.
- The new message when an user assign to himself an issue or a mr is: "You
assigned issue xx to yourself"
- The new message when an user adds a todo to himself is: "You added a
todo for issue xx"
- The new message when and user mention himself is: "You mentioned
yourself on issue xx"
- The new message when an user directly address himself is: "You
directly addressed yourself on issue xx"
- The new message when an user mark himself as an approver is: "You set
yourself as an approver for merge request xx"