Put an upper limit on UTF8 file sizes

This is for #7223

* Start the discussion at limiting files to 4MB and under before we refuse to load them
* Display a message if we've reached the limit opening a file
* Implemented in AppShellFIleSystem which is really the only place this could go without rearchitecting a lot of file system work but looking for feedback there too
@redmunds I guess I'd err toward the lower number given that we know 68MB causes a crash and afaik we haven't tested any sizes between 5 and 68, so we don't know where the threshold falls...