Login to Observe API

[batpad]

Implements a button on the Settings page to Login to the Observe API:

• Needs OBSERVE_API_URL to be set in your .env file.
• It will open the API /login URL in the default phone browser
• After the user logs in, they are redirected to the app, and an action is fired that handles adding the token to the state at state.observeApi.token

We can then use this token to make authenticated calls to the API. I'll work on the boilerplate for a service to use the token and talk to the API in a separate PR.

TODO:

• Get Linking working on iOs, following iOs docs at https://facebook.github.io/react-native/docs/linking - @geohacker
• Write simple tests for action and reducer

[batpad]

Amazingly, this seemed to work on iOs without any changes needed, since we already had the iOs stuff for Linking wired up.

@geohacker @sethvincent can you give this a look? I think this should be good to merge.

Fixes #116

[geohacker]

@batpad this works well on iOS and Android for me. And I also checked along with OSM login. Seems all good 👍