This modifies displayed power levels such that:
 - If users_default is !== 0:
   - the power level 0 is displayed as "Restricted (0)"
   - the power level users_default is displayed as "Default ({users_default})"
 - Otherwise:
   - the power level 0 is displayed as "Default (0)"

When changing users_default, to say, 10, when the textual powers are rendered
again, they will take users_default into account. So those previously at 10
and which would have previously have been rendered "Custom of 10" will now
read "Default (10)". Conversely, those that were "Default (0)" will now read
"Restricted (0)".

…-restrict-power-level-options

Also fix not-i18n-friendly "of" to be "=".