insights: frontend: need indicator if backend is populating historical data or not

Created by: slimsag

As the backend walks slowly back in time and looks for data, there are a few states that the frontend could render better:

1. Directly after adding an insight, there may be no data points. Currently the display for this is quite ugly (empty chart, looks broken:

2. We may also have only a single data point for an extended period of time. For example, if the data backfiller is hard at work on other series, it may not get to yours for a few hours or so. In this case, you'd be stuck looking at the singular data point that the regular worker records every 12h:

3. As historical data is being found, it does so repository-by-repository. This means that the oldest data point returned may be incomplete. For example, if looking at this global view, the 10/13 data point is incomplete - one option is for the backend to not return this, another option is for the frontend to warn the user that it is an incomplete data point (always the oldest iff historical data is being built still):

TODO:

• Decide exactly what we want to display here on the frontend.
• Expose this type of "loading" / "progress" information from the backend.