Changes:
- Resolved the issue of safely deleting categories and updating the events
  that used to be classified by those categories. Essentially what now happens
  is that the deletion of a category triggers an immediate update to all
  events of that classification to now have the string "No category selected"
  as their category description. This works in all GUI views.
- Deleted category added message that does not disappear because the user can
  readily see that a new category was created and the user should only be
  informed in cases that are either warnings or non-visible changes.
- Addressed the issue of a null populated category list when creating an event
  from within the EventEditor class.

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Changes to be committed in next commit:
- Set the submit button in categories to a deselected state when there are
  no categories present.
- Set the apply filter button in categories to a deselected state when there
  are no categories present.
- Make a change to the EventEditor class so that it will conditionally redirect
  the user to the EventUpdater class upon the choice of the user when the
  event that the user is intending to submit has the same name as another event
  that already exists.
Merge branch 'dev' of https://github.com/tomorrow-nf/wpi-suite-team5 into dev