Disable 'Mark all as read' for notifications when everything is read

- This action needn't be performed when all notifications are read,
  hence disabling it is a good practice
- Added a link-disabled-color
- Used css pointer-events to disable clicking on the link

Preventing default on click for the mark_all_read_link

- avoiding the use of pointer-events
- setting up disabled only for mark_all_read_link

Moved disabling of the link to a global handler

- Added disabled class to the button in /notifications when all are read
- Styled the disabled button
- Adding disabled class after completion of ajaxRequest
Merge branch 'ssrihari-disable-mark-all-read' into develop