Add tooltips to the "Block Details" page

[andogro]

Tooltips will help users understand the presented data. These are suggested values for the Block Details page, and may vary according to the selected chain. Unsure of best design strategy (hovers, additional section to left or right etc.) @pashagonchar is currently working on tooltips for other sections (see #1726) and will have design insight.

Based on #1764 may also add hover over for timestamp.

Suggested Additions

Value	Tip
Block Height	The block number in which transactions were recorded.
Hash	The SHA256 hash of the block.
Parent Hash	The SHA256 hash of the parent block.
Difficulty	Block difficulty for miner, used to calibrate block generation time (Note: constant in POA based networks).
Total Difficulty	Integer of the total difficulty of the chain until this block.
Nonce	64-bit hash of value verifying proof-of-work (note: null for POA chains).
Uncles (if displayed)	Index position(s) of referenced stale blocks.
Gas Used	Total gas used by all transactions in this block (% used / limit).
Gas Limit	The maximum gas allowed in this block.
Miner / Validator Block	If Miner: Block Miner's Address. If Validator: Block Validator's Address.
Block Rewards Block	If Miner: Amount of distributed reward. Miners receive a static block reward + Tx fees + uncle fees. If POA: Amount of distributed reward. Currently validators receive 1 POA token + TX fees, and the Emission fund receives 1 POA per validated block. If xDai: Validators receive Tx fees if collected (If transactions exist in the block)