Add /input page

[casey]

• Test empty scriptsig rendering
• Test Server::input with coinbase transaction
• Test block, transaction, input not found
• don't render null previous output
• don't render max sequence
• don't render empty witness
• Make titles monospace (anything else?)

[casey]

@raphjaph This lets you view inputs by going to /input/PATH, i.e. /input/BLOCK/TRANSACTION/VOUT.

Once runes are in a format that they can be embedded in inputs, we can parse out and display runes on this page.