Add more complete test coverage

[andrewda]

Just to make sure we don't break things 👍

[machour]

We really need tests for 1.4 development, the app is getting bigger and harder to test by hand

[andrewda]

@machour Here's the blocker: RealOrangeOne/react-native-mock#141

[machour]

Tried to add jest tests for GithubHtmlView earlier but got that exact same problem :(

[andrewda]

@machour I got it working using a fork of react-native-mock in #148.