`generichide` option is implicitly enforced on all sites unless an
exception overrides `generichide`. Though rare, sometimes a
`generichide` exception needs to be overridden so that generic
cosmetic filtering is made possible on a specific site.

This commit is to add support for restoring generic cosmetic
filtering on sites which were excluded through a `generichide`
exception.

Concretely, this is needed to ensure the test suite can properly
verify that generic cosmetic filtering is working when the
filtering mode is set to "complete":

||ublockorigin.github.io^$generichide,important

@description
Conditionally prevent assignment to `innerHTML` property.

@param [selector]
Optional. The element must matches `selector` for the prevention to take
place.

@param [pattern]
Optional. A pattern to match against the assigned value. The pattern can be
a plain string, or a regex. Prepend with `!` to reverse the match condition.

As discussed with filter list volunteers.

Related issue:
uBlockOrigin/uBlock-issues#3582