Adding support for hashlink images

Hi all,

with Blockcerts v3 we have added to option to support hashlinks in the display html property.
The reasoning behind this addition is to limit the size of the final certificate as we were mostly relying on base64 images until now.

It has a downside that the images or documents now need to be hosted for as long as you expect your certs to live.

You would for instance use hashlink like this:

<img src="hl:zQmX1YwVQab8fjMUUEzzBkhpMp47F5HNa4PQacCTdi11WDo:z4S6gis45zzpJLgobGDmWfZE1toUseAuoV19VWairCZJQ9L4uNC1ot2wEbZX1DngU25aMPeRH47FH7uRNo6rAPiYSYXXwWzYaZ1NepxJb7ERCP2vMjFJVRKB9eginr18kA1bSMqPHHhmWNqp1WMDt2oLxMoDG9TC" style="max-width:100%;height:auto;width:inherit;display:block;width:612px;display:block;box-sizing:border-box"/>

Instead of using the base64 encoded data.

The image is going to be discovered and parsed, the url updated in the display property so that the expected image displays. When starting the verification process, the hashlink is going to be verified against the hosted image. If the hash matches, then verification is successful, if the image has changed then verification fails.

Here is more detailed information about hashlinks: draft-sporny-hashlink-07

To sum up:
pro: lighter blockcerts
con: requires hosting images “forever”. Certs are not self-contained anymore. It is not compulsory to use hashlinks.