Redesign how short URLs are generated

[cdchapman]

Helpful background documents:

• https://www.bl.ink/blog/url-shortening-best-practices/
• https://www.bl.ink/blog/bl-ink-basics-urlbuilder/
• https://www.bl.ink/blog/what-is-enterprise-link-management/

Business requirements:

• Blog posts only need short URLs once they are published. But we should use a different short link for each version, so we can track links to individual document versions (see Create a process for snapshotting items #117). This should be automatic.
• Should be able to add arbitrary links to track specific marketing campaigns, etc. (does the insertion need to be dynamic? I think not, since marketing campaigns are going to be a pre-thought-through sort of thing, and may include additional syndicated content that we would publish at the same time).

Implementation:

• Update short_url helper code to implement new short link design. - Do we use a third-party shortener, or roll our own with something like NewBase60? Whatever it is, it should be deterministic.
• Add nginx server for the short_url domain (if it exists).
• Express link map as nginx redirections.
• Create a Nanoc data source and accompanying file that creates short url items for arbitrary URLs.
• Create Nanoc rules for these url items.
• Add tests.