Manage colors for locations in analytics charts

[McPringle]

Switching the years in the analytics charts on the dashboard changes the colors of the bars of the locations. In one year, Luzern is blue, in the other year, Luzern is green, or red, or whatever. Stepping through the statistics of the years makes recognizing the locations difficult if the colors are constantly changing.

Add a settings page to the admin, where custom colors can be configured for each location. Use these fixed colors for the bars on the analytics chart, so the same location has always the same color.