Fix account create checkbox bug

Fix account create checkbox bug

When a block is being created on Special:Block, the "create account"
checkbox is checked by default if the block is set to "Sitewide" and
unchecked by default if the block is set to "partial".

There are multiple scenarios where the checkbox should not check and
uncheck according to the type of block.
1.- When editing a block. This means that an admin already saved the
    the block so we shouldn't mess with the options
2.- After posting the form. An admin was ready to save and it didn't
    happen for whatever reason. Still we shouldn't mess with the options
3.- After manually clicking the checkbox itself (already working before
    this patch)

Bug: T216845
Change-Id: I39b723d1f638d016c74ec20639efeecac95d8cdd