Support "abp:" and "subscripe.adblockplus.org" filter list subscription URLs

[mario]

Background

In order to allow users to subscribe to unofficial filter lists (e.g. EasyList Spanish) support for filter list subscription URL's needs to be added.

What to change

Add filter list subscription prompt

1. As soon as a filter list subscription URL is tapped, prompt the user whether they wish to add the filter list to their filter list subscriptions as shown in [this screenshot].

Prompt text: TBD
Prompt confirmation text: TBD
Prompt cancellation text: TBD

1.1 If the user cancels the prompt, close the prompt and don't perform any actions
1.2 If the user confirms the prompt, add the new subscription to ABB's default filter list subscription as described under "Add new filter list subscriptions"

Add new filter list subscriptions

2. Add a new list component to the "Ad blocking" settings page (Settings->Adblock Plus->Ad blocking) as shown in [this screenshot]

Headline: TBD ("Custom subscriptions")
Content: Empty by default
Visibility: Hidden by default

2.1 As soon as one or more custom filter lists are subscribed (3.), automatically show this element as shown in [this screenshot]
2.2 As soon as all custom filter lists are removed (3.), automatically hide this element as shown in [this screenshot]

3. As soon as the subscription of a new filter list has been confirmed (1.2), add a new filter list item to the list new custom filter list component (2.) as shown in [this screenshot]

Text: <filter list name>

3.1 Enable newly added custom filter lists by default
3.2 TBD: Enable users to remove custom filter list items

Additional information

In addition to 1., filter list subscription URLs either start with "abp:subscribe" or with "​https://subscribe.adblockplus.org" as documented at #2208.

[mario]

Will add screenshots, style guide and missing information as soon as assets are ready.