Make blacklists part of the settings

A few remarks:
- I used the unfortunate name `blacklistedsites` since `blacklists` was apparently an old existing name
- The import from existing `BLACKLISTS` should be double-checked (I tried, but…)
- The discrepancy between toggle and check remains (one uses @ match the other doesn't)
- I tried to stay close to your coding style, but I'm not really familiar with JS, so please change as you will
- I tried to update the docs too (resulting in two commits that you can squash if you prefer)