When enabling an extension, we'd mark the RegisteredExtension entry as

installed and then save it. This was only really necessary the first

time an extension was installed, but we were still doing it every time.
This change simply makes the update conditional, and reduces the expense

of the save by specifying that only the installed field should be

written to the database.

Unit tests passed.
Manually tested enabling and disabling extensions. Verified that

extensions were always loaded when they should be and would reload when

next initialized.