Adapt code to work with subscriptions.json

Background

See trac#7371

What to change

• Rename references to subscriptions.xml to subscriptions.json
• Update code that uses subscriptions.xml to parse and access it as a JSON object
• Update subscriptions.xml file in repo with subscriptions.json

See also integration notes.

Hints for testers

• Recommended filter lists should be displayed correctly in the desktop and mobile options page.
• Selecting, adding, updating and removing recommended filter lists remains unchanged.

Recommended filter lists are language filter lists (e.g. EasyList Germany) as well as built-in feature such as EasyPrivacy.