Set mention notification as read when viewing post

  - If a mention for the current user and the displayed post exists,
    mark it as read
  - Use set_read_state instead of manually setting unread as false
Add test for mention notifications
Set sharing notification as read

  Fix issue #4951

  If the displayed user (on her profile) started sharing with the
current user, mark the notification as read
Merge pull request #5009 from taratatach/4951-sharing-notification-not-marked-as-read-when-visiting-profile

Set sharing notification as read
Merge pull request #5006 from taratatach/4940-mention-notification-not-marked-as-read-when-viewing-post

Set mention notification as read when viewing post