Charts v0

Introduces the following functionality in the chart tab:
- New chart button with a dropdown filled with Question Labels according to form.json. Clicking on these is non-functional at the moment.
- A submission-time chart appears when the chart view is loaded.
- Trivially (by reverting [this commit](https://github.com/onaio/zebra/commit/c8147b2a516e19ead77b685dce995e6c2688ca2b), all charts can be loaded, rather than just submission time).
- A URL, dataset-id/chart/field_name is added, which will produce a single chart, just for that field name.