Investigate treatment of whitespace in route parameters

Brought to our attention via a production bug #4982: at least this badge accepts trailing spaces in the route parameter. A brief discussion ensued at https://github.com/badges/shields/pull/4983#discussion_r418226181.

I imagined, and still do, that _most_ of our routes should be limited to characters like A-Za-z0-9_-. Most usernames, project names, etc. cannot include whitespace.

We may want to think about a global change to the default route-component regex, with overrides in places where whitespace is acceptable.

Start by reviewing production bug #4982 and the discussion in pull request #4983, which provide the current evidence about trailing spaces in route parameters. Determine the intended default route-component behavior and where exceptions would be needed; the issue is complete only once that scope and approach are settled.

Written by the indexing model from the issue text.