Change legends and tooltip labels of block complexity, add annotated explanation

[nmarchio]

Alias the legend labels for the block complexity field to show something like the following (including an annotation footnote at the bottom). https://github.com/nofurtherinformation/mn-explorer/blob/master/config/MapVariables.ts

Block street access *
1 
2 (high)
3
4 (moderate)
5
6 (low)
7
8
9
10+ (very low)
Off-network
________________
* measures the number of building parcels between the least accessible structure and the street network. This metric is called block complexity.

Change block complexity tooltip labels to "Building street access": https://github.com/nofurtherinformation/mn-explorer/blob/master/config/TooltipColumns.ts#L21 https://github.com/nofurtherinformation/mn-explorer/blob/master/config/TooltipColumns.ts#L83