Add url to FBbi imaging method ontology

[dominikl]

Simply adds a link to the FBbi image method term. Like its done for Data DOI, Annotation File, etc.
/cc @francesw @sbesson

I'm not sure if/how this can be transformed into using icons like it's done for the mapr annotations...

[sbesson]

I thought I had opened a PR to that effect in the past but apparently not. To turn these URLs into icons, I think there are two things that need to happen:

1- split the value into separate keys (Imaging Method and Imaging Method URL) - see e.g. sbesson@07d577b
2- update the mapr configuration to register these keys and allow them to be processed together

If we decide to go down that route, I could imagine more of the keys under the idr.openmicroscopy.org/study/info namespace can be processed in the same way. This would definitely have some breaking implication for anyone who would parse these values by hand but long-term, it would be an improvements in terms of usability and searchability.