Add a way to disable custom filters

Background / User story

While it'll take a while until we have proper support for multiple custom filter lists, we could at least provide users with the option to easily en-/disable their custom filters all at once.

Initially, we intended to add this feature as part of the new custom filter table (see #15 (closed)) but, due to the fact that the filters in that table are actually spread out over multiple custom filter lists, there was no simple way to disable all custom filters.

See https://issues.adblockplus.org/ticket/315

What to change

• Design: TBD
• Research: TBD
• Spec: TBD (see relevant section in old spec)

Hints for testers

TBD

Hints for translators

TBD

Integration notes

TBD