Change time horizon for Avg Time per Block and Avg # Tx per Block

[wy]

Current behavior

• Do this after Add Avg # of Txs in block as a Happynodes metric box #98
• Currently calculates avg time per block based on a fixed number of blocks
• Make this more flexible

Expected behavior

• Since this is an API call and API calls are generated indirectly through SQL, avoid excessive calculation by only allowing a certain number of degrees of freedom e.g. Last 5 blocks, Last 10 blocks, Last 100 blocks.
• Add this to a new page /blockstats rather than in the top because that will clutter the UI