modules next: added basic item editing modal

test plan:

1. click the item row cog
2. choose “edit”
3. change the title
4. click save
   - item indicates “saving..”
   - item’s title updates
5. refresh the page
   - changes persist
6. repeat 1-3
7. click cancel
   - title does not update 

Change-Id: Idcd52c580078ca60ec468a4c85832b385d63220c
Reviewed-on: https://gerrit.instructure.com/38487
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Ryan Florence <ryanf@instructure.com>
QA-Review: Ryan Florence <ryanf@instructure.com>