Prefixes.js is not very robust (this may be policy as much as code)

[DILewis]

The prefixes.js file sets up an object which associates prefixes with URL expansions to make typing and reading URLs in the code easier. This is declared as const.
A few observations:

• Individual apps may also need extra prefixes. These don't have to be added to a central object, but that might be useful
• Apps may rely on these expansions, so, e.g. we should probably prefer adding entries rather than changing http->https so that existing prefixes continue to provide backward compatibility
• Perhaps we can place these somewhere where they are easier to include

Note on motivation of this issue: We have, in the Beethoven in the House project an example where the traverser has been encapsulated within Vue native objects. In this use case, the devs felt it was easier and more robust to duplicate the prefix object and then extend it. This prompted discussion of what good, sustainable practice would be and how we could support it.