Clickable scientific topics

[eilmiv]

Summary of changes

• make operations and scientific topics clickable when showing event, material or learning path (updated _extra_metadata.html.erb)
• make operations and scientific topics clickable when rendering as tags on material and learning path lists
• applied css classes for each type of tag on the material (or learning path) list to allow individualized styling by custom themes (tag-topic, tag-operation and tag-keyword)

Motivation and context

Closes #1250. This came up in the context of linking to training resources in PaN Training (a TeSS based catalogue) from PaNET ontology terms.

Screenshots

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.

[eilmiv]

This contains also some minor changes from automatic code formatting especially in the modified test files. I'm happy to remove them, but I agree with most of them, so I left them in.