Fix issues from user feedback

Fix the following small problems found during internal acceptance and user testing:

• Cover image works if uploaded, but before uploading it, it should be the same as the thumbnail image, currently it is always the default image.
• System always asks if the user is sure they want to leave if the exhibition has not been saved, even if no changes were made.
• Publishing an exhibition should also save it.
• The save changes button in the details editor should be a different color, not white.
• The back button should have a bigger hitbox.
• Permanently deleting an exhibition needs to be refreshed before seeing the change.
• Back button should go back, not always to home page.
• Activities dropdown should open on hover and be bigger.
• Exhibitions created twice sometimes.