Improve specs and prototypes index page and mechanism

Some ideas of what could be improved in the specs and prototypes index page:

• Filters: text and type (spec, prototype, HTML page)
• Sorting: created date (default), alphabetically
• Button to easily copy URLs
• Links to the issue/epic/merge request referenced by the deliverables
• Optimize script to no longer require appending -spec-previews Fixed: #638 (comment 229048772)
• Making it easier to share uploaded specs:
  1. Use a Git hook (like pre-push, post-commit or prepare-commit-msg) to:
     1. Print the full URL of the specs to the CL
     2. AND/OR automatically copy that URL to the clipboard (see https://stackoverflow.com/questions/39692617/copy-a-string-to-clipboard-from-mac-os-command-line)
  2. Use CI to automatically add a comment on the referenced issues/merge requests (or in the commit itself) with a link to the specs.