Allow tracking additional criteria, not just Category & Priority

[gapple]

There are other metrics that may be informative to track, even if they aren't factored into the release estimation, such as beta blockers or the core initiatives.

1. The current samples table doesn't support additional criteria well, since it requires a new column for each value that is tracked, and should be normalized with a samples_values table that stores key-value pairs for configured criteria
2. The current code makes some assumptions based on only Category and Priority changing between fetching values, and should be changed to isolate all parameters between each request.
3. Configuration is currently in static class variables, and should be moved outside of the class and injected instead.

[gapple]

Fixed in 4ff5616..9ba9580