Add task instructions to UI

[planemad]

After using the HOT tasking manager, having a task briefing and instructions makes it easy for beginners to try out the tool and learn at the same time.

This is also very useful even for the Mapbox data team to document things like background tile layers to use, what features to focus on, what to ignore and documentation to commonly used tags for a task.

@aaronlidman I have written a sample briefing for krakatoa: https://gist.github.com/planemad/1c71b237d8a1fedc25f3

For now it could be easily added as another tab Instructions before Task and Activity.

[rasagy]

Similar to #63. Would make sense to show a modal when you open a project for the first time/switch to a project, and make the modal also accessible on demand from a More info / (i) button. cc @ajithranka