StatusHistory: Consider replacing with Heatmap

 StatusHistory is a weird panel. We originally figured that when we'd rewrite the Heatmap panel core to the new architecture that we would drop support for a non-numeric Y axis, so we needed a panel that could still represent discrete status, regularly sampled over time for multiple series, with series names on the y axis, without any kind of bucketing or aggregation available in the panel itself. Something like https://github.com/flant/grafana-statusmap. Where we ended up is rather different. When we eventually rewrote the Heatmap panel, we decided to go for 100% parity with the original so we could just remove the old implementation and auto-migrate users without any kind of manual user action or regressions. This went surprisingly smoothly :tada:. Except now we have two panels that _nearly_ do the same thing when Heatmap panel is in its default `Calculate: No` mode (a behavior change from the previous Heatmap).  The goal here is to remove unnecessary, similar-looking panels for users to choose from, and remove multiple rendering cores. To make this possible, the main additions we need to Heatmap are: - ~~Ordinal x rendering (StatusHistory does not render on a real time scale, all cells are equidistant)~~ - Color cells from threshold or from value mappings - Cell can be stroked, not just filled - Value text rendering in the cells - Additional legend format? - Some work for allowing cell gaps to to be controlled in just x direction or just y - Add option to not auto-skip tick labels due to row density