Noissue - Added documentation about test environment
This MR adds some documentation relating to usage of our mock environment. The reason for it being that the documentation in our README is quite outdated and that we're going to remove it from there as part of #262.
Some parameters in there are marked as "deprecated" since they're still in the code but are no longer relevant while others (e.g. "blockedURLs") are still in background.js but not used anywhere anymore and therefore not included in the documentation anymore.
There are also some global parameters (e.g. "application", "addonVersion") which are available to all UI pages and used on some of them. I decided to only mention them in places where they allow testing of functionality for which there's no dedicated parameter yet.