create input validation function for tasks

[jordan-manus]

When testing, i am finding that the user can add the same task multiple times in the new_item() and update_item() functions.

1. Go to the main page after login successful
2. add a new item to your list (i.e.: sweep)
3. add a second new item to your list (i.e.: sweep)

the same error is present when a user goes to update an item:

5. select one of your duplicate items and click 'update item' button
6. retype the original duplicate task (i.e: if you have 2 sweep tasks, edit one of them and retype 'sweep')
7. the duplicate sweep task is added back to the task list
   
   
   

[ekaulfuss]

As intended. If a user wants to do a task multiple times they can. Consider: If a user wants to remind themselves to drink more water through out the day they can add the task "Drink Water" 8 times then delete them as they drink water throughout the day.