`/visualblocks` page does have a tab on the dcrdata UI

We could have a link/tab to the [visual blocks page](https://dcrdata.decred.org/visualblocks)  on the `/blocks` page as we do for [Windows](https://dcrdata.decred.org/ticketpricewindows).

> Incorporate visual blocks. Not on the homepage. Maybe a /blocks page option?
Ref: https://github.com/decred/dcrdata/issues/1191#issuecomment-476683415