Resolves #854

Essentially returns rows of the addresses table with CSV-formatting and appropriate headers.
It is surprisingly fast, even for the treasury address which returns a 46 MB file.

The rows returned are

1. tx_hash
2. valid_mainchain: [0,1]
3. matching_tx_hash: can be empty
4. value: dcrutil.Amount(value).ToCoin()
5. block_time: uses TimeDef.String() YY-MM-DD HH:MM:SS format, but should maybe be either a timestamp or an ISO formatted string
6. direction: [1,-1] Indicates whether this is input or output of the associated transaction. +1 for output (increasing address balance), -1 for input.
7. io_index
8. tx_type: from txhelpers.TxTypeToString(txType)

Potential areas of discussion

1. There is no block hash (or height) column, but if it is needed, I could perhaps do a JOIN with the transactions table.
2. This introduces a new route in apirouter at the /api/download path. There may be an argument for creating a new router at /download instead.
3. The auto-suggestion for the download filename is not customized in any way. It is always 'address-io.csv'.
4. Nothing is paginated in any way. You get everything every time. Though as mentioned above, everything seems to run quite fast.

One more note. I'm not sure why the binary icon font files got smaller when I added an icon. All of the icons seem to still be there.