contact: add online encryption tool

Add a simple online encryption tool which allows cut-and-paste encryption of secrets.

The change can be seen in action at https://guidebook.devops.uis.cam.ac.uk/en/make-encryption-easier/contact/public-keys/ which has been validated by getting the Collaboration Tools team to send Azure credentials to us.

Closes #115 (closed).

changed milestone to %DevOps Sprint 133

assigned to @rjw57

approved this merge request

requested review from @rjg21

merged

mentioned in commit 2bfab60b