Address reuse charts

[xaur]

Utilizing dcrdata's indexing infrastructure, it might be possible to calculate the count of addresses that have been used previously, for each block. This can be counted for staking and non-staking tx separately. The numbers can then be put on a bar chart with block and day bins.

See also: https://github.com/RichardRed0x/decred-research/issues/2

[xaur]

This chart can be used to monitor the amount of address reuse over time, which is a proxy to network's overall privacy and security. Specifically, when the ticket-based VSP is rolled out, we should see a pretty decrease of address reuse.