Adding the 'allow comments' tickbox in textbox config (Bug #1037531)

Currently you have to go to Content -> Notes to turn it on which seems
like excessive clicking when you can just activate it here.

Also we will have allow comments ticked on by default

Change-Id: I1efc6d6c65e2c1f6eddec5f19a5a4fbb6e471fec
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Merge "Adding the 'allow comments' tickbox in textbox config (Bug #1037531)"