CSV address data download.

[buck54321]

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.

[chappjc]

I would make time_stamp a unix epoch time stamp, since it is compact and unambiguous.

The PR description has a little copypasta: tx_type comes from txhelpers.TxTypeToString(txType) and value from Coin.

I'd also suggest putting direction either right before or right after matching_tx_hash, and with io_index adjacent to matching_tx_hash too.

I think there's a pretty strong argument for changing /api/download to just /download with a new router, primarily since the content-type is different.

For the download file name, some kind of current time stamp seems appropriate, perhaps with a best block height.

How were the ttf and svg files edited? fontforge and Inkscape? The file size changes are peculiar.

[chappjc]

Oh, it looks like a thumbs up and thumbs down glyph were removed from the ttf.

[buck54321]

New header sequence is "tx_hash", "direction", "io_index", "valid_mainchain", "value", "time_stamp", "tx_type", "matching_tx_hash".

Server's filename recommendation is address-io-[best block height]-[hex encoded timestamp].csv.

New path is /download/address/io/{address}.

re: font file stuff. I guess I can't say for sure, but it looks like the fonts were previously created with IcoMoon, so that's what I used too. IcoMoon generates the 4 font files. Unfortunately, this means that I can't tell you why the decimal precision changes, but it looks like it only affects 3 numbers and not the entire SVG. I think the thumbs up/down icons were extraneous because I can't seem to find a reference to them in the code. Perhaps someone previously pruned the icon set but didn't replace the TTF.

@chappjc I didn't change the address validation because I'm pretty sure that it's right, but could you check.

[chappjc]

The address validation is good for sure, I just wanted to call out that there are a lot of non-nil error values that are not unexpected. I wanted to be explicit about my motivation for changing to Debugf from Errorf.