Cache db adapters

[mpsijm]

Will resolve Bastiaan's feedback.

• MainController now contains a map from user to another map, that maps from prUrl to database. In this way, databases only have to be created once.
• I also decided, to make things easier, to log the start/stop watching PR events mostly in the content scripts on window focus and blur. Saves us a lot of edge cases to consider in the MainController concerning switching tabs.

[coveralls]

Coverage decreased (-0.9%) to 92.176% when pulling b3dce56b3b8c8228db2c4bcd742b2df6ff8ec173 on xx-cache-db-adapters into b5226ed4160f738991810400a199b8ad7c475974 on dev.

[coveralls]

Coverage decreased (-1.1%) to 91.942% when pulling 7f8b4bf86d303dee1b2eb63186d81cfa1ea78003 on xx-cache-db-adapters into b5226ed4160f738991810400a199b8ad7c475974 on dev.

[coveralls]

Coverage decreased (-1.1%) to 92.086% when pulling 383e5c4864daf7e08c3420d112dddefcef59c6d5 on xx-cache-db-adapters into 998d5d03c65f47632610c8ef878cc8c577dd730a on dev.

[mpsijm]

Coverage decrease is because I mostly edited MainController and ContentController. All other things are still tested.

[coveralls]

Coverage decreased (-1.1%) to 92.091% when pulling 3e88414f11e74e9abf4aee4675b9ee90d5c69794 on xx-cache-db-adapters into 998d5d03c65f47632610c8ef878cc8c577dd730a on dev.

[coveralls]

Coverage decreased (-1.1%) to 92.091% when pulling d2ab262d6e9eef695303cb8e24cd5f6435b54013 on xx-cache-db-adapters into 998d5d03c65f47632610c8ef878cc8c577dd730a on dev.

[Exclaminator]

LGTM, if the tests succeed Ill merge.