General:
-  Update configuration files for running tests properly (tsconfig and webpack)

Fix tests for the following components:
-  Content
- Root
-  Launchbar
- BrowserNavIcon
- BrowserNavBar
- Header (note: deleted a couple of tests)
- HeaderButtons (note: skip test for calling the toggleAccount function; unsure about current behaviour of the component)
- BrowserBar
- App

Delete useless tests:

- Browser (remove snapshot test; will try to come up with more useful tests)
- BrowserImage (current tests don’t do anything useful; proper testing will be difficult, because this component extensively uses custom events)
- LaunchbarContainer
- LaunchbarIcon (they were copy-pasted tests for LaunchbarContainer anyway)
- browserActions and headerActions (tests were essentially testing the library we use to generate actions; and we shouldn't be testing third-party libraries)
- headerReducer and browserReducer (reducers are very simple and hardly need testing)
- browserSelectors and headerSelectors (test are very simple and testing things that Typescript will report anyway)