This moves the way we add the user avatar and site logo
to the discobot certificates from embeded base64 png to
just using the files urls in the href to the image tag.
This will make generation faster and the certificate
smaller overall, but it can't be used in a `img` tag
anymore, since SVGs in `img` tags don't load the external images
In order to work around that we will move the certificate
in posts to an iframe, which works fine without any user
visible changes. For this to be possible the plugin automatically
adds the site current domain to the list of allowed iframe origins.
FileHelper.download requires a string not a URI. I also found another
instance of using open-uri directly and swapped it out to use
FileHelper.
I also updated it to not `read` a file if it comes back nil.
Follow up to: fe01099a38
To eliminate a DDOS attack vector, we're taking the following measures:
The endpoint will be rate-limited to 3 requests every 60 seconds (per user).
A 24 hours max-age cache header is sent with the response.
The route will be hijacked to generate the certificate in the background.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
