Cache static files by hash

Implements #2734.

Deployment note: `GRATIPAY_ASSET_URL` needs to be changed to `https://assets.gratipay.com/` on Heroku. I don't think we'll need to change anything on the CDN side, can you confirm that @whit537 ?
Ok. Then not cmd.exe definitely, because is there `/` is an option delimiter.
```
> dir src/blockdiag
Parameter format not correct - "blockdiag".
```
But this stuff worked ok:
```
> dir "src/blockdiag"
```
Might be a PowerShell or Clink feature, though.