key endorsement: ad fd_comments, link to admin view

We are sometimes replying to key endorsements on nm@d.o, stating that we don't like the endorsement or whatnot. That information should also be on nm.d.o itself. Therefore I propose:

1. adding fd_commends to the FingerprintEndorsement model (similar to others)
2. enable the admin vie of FingerprintEndorsement
3. add a link in the https://nm.debian.org/fprs/person/$person/$fpr/endorsements/view/ to the admin view
4. display the eventual fd_commennts in the endorsement tables (which I noticed are duplicated between fprs/templates/fprs/view_endorsements.html and process/templates/process/req_keycheck.html - the table should probably be a separate template file included in both).