Bug 858118 - [system] fix and reenable unit tests for update manager r=etienne

- fix updatable tests and use the new mocks helper
- reenable update tests without any more change as I couldn't make them fail.
Feel free to disable again if this intermittently fails after all.
- fix lockscreen test for various firefox versions
(cherry picked from commit 3f9979fe26842d90c01e040f57edb9ff59580766)

Conflicts:
	apps/system/test/unit/lockscreen_test.js
Merge pull request #8787 from lightsofapollo/bug-839273-add-gaurds-for-removed-headers

Bug 854292  - Add more safegaurds for headers being deleted while they ha...(cherry picked from commit c3a1298ff0d6ec23d0f1aee4f195be7ffd6ca2fa)
Merge pull request #8983 from jrburke/worker-thread

Bug 814257 email: Move back-end into worker thread, r=asuth, r=jrburke, r=lightsofapollo, r=squib(cherry picked from commit 0649e5e9d2f71c96517bca1b582c706e665bca6a)