dashboard: support all API signals

[sgratzl]

It would be great if the new generic dashboards supports all COVIDcast API signals

[sgratzl]

we need certain meta data about the signals such that they can be used in a useful way. The COVIDcast 2.0 is focused on comparable signals (per 100, per 100k, ...).

Information needed:

• highValuesAre: 'bad' | 'good' | 'neutral' ... influences how the information (trend, color scale) is presented
• name ... use user interpretable name
• rawSignal ... link between smoothed/average signals and their raw signals
• unit: raw | percent | per100k | fraction ... influences how chart labels are shown
• tooltip, type (public | early | late), ...