`Blocks` should be renamede to `Slots` throughout the site.

[MicahZoltu]

The consensus (beacon) chain has slots, the execution chain has blocks. For clarity of communication, most instances of Blocks on the website should be renamed to slots. Not all references should be changed though, because blocks still exist for the execution blocks.

[MicahZoltu]

For example, https://github.com/gobitfly/eth2-beaconchain-explorer/blob/845f48ab5bc1e30793e2c3ec7c31a272bb03c1b2/templates/blocks.html#L12 should be changed to /slots/data and most (maybe all) references to blocks on that page should be changed to slots. Similar for nav bar, blocks list page, etc.