Update frequency and consistency of coverage

[wafels]

Some previously missing images were added to the image store today. There are three different places where image coverage is described. These are

1. The image timeline in the viewer window.
2. The coverage statistics plot - https://api.helioviewer.org/statistics/bokeh
3. The coverage page - https://api.helioviewer.org/coverage

Some questions:

• Do these all give the same results?
• What is the update frequency of these results?

See also #50