Match colors in graph with labels

Made selectedProjects hold objects instead of strings
Added a color property to these new objects
Added a colorScale observable that's set by the vega component
Made WikimetricsVisualizer listen to the colorScale changes and update
the colors on each selected project.

Removed prettyProjects from what the API returns.  The pretty project
names are accessible from the reverse lookup's values.

Standardized naming.  Things like 'enwiki' are always called 'databases'
and things like 'wiki' are called project codes.

Change-Id: I5d956528e16462316c1394d6c4bbcb6273f28d7e