CodebyRay Flarum should work the same on both www.mydomain.com and mydomain.com to avoid issues with site visitor.
I agree, that the www-version should work even if your website is meant to be without www and vice versa. But it shouldn't work the way you probably think of, i.e. both versions shouldn't show your forum and thus the same content. One reason for that is the simple fact that search engines don't like duplicate content. While the search engines might be satisfied with a canonical link element, this is not the preferred option.
CodebyRay I did, however I think this should be fixed on the system level with Flarum.
I disagree. This has nothing to do with a forum, it affects every website and should be handled separately. As luceos pointed out, websites may have very different requirements for their redirects and automating one part of it may negatively affect other use cases.
At some point people installing Flarum will have no clue about doing a redirect to solve the issue and may just remove Flarum and move on to the next forum before giving it a chance.
I would suggest giving relevant hints in the installation instruction. One might even include a sample .htaccess
file for apaches and directives for other server softwares to the same effect. But in the end it's the site owner who should get at least a basic understanding about the things going on.
There is, however, one thing that might be changed in the core software, which doesn't solve the problem of duplicate content but remedies the broken links in the www-version of your original post. My suggestion: Replace the domain including link for the stylesheet, in your case:
<link rel="stylesheet" href="http://motorcycledealerforums.com/assets/forum-920ce024.css">
with one without the domain:
<link rel="stylesheet" href="/assets/forum-920ce024.css">
In this case, the observed trouble with the FontAwesome font should disappear.
Are there any side effects stemming from this approach?