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
@JeffryBooher If we want to enforce coding guidelines like that then maybe we should have different limits for different files types? I would only expect a .log or .json file to ever get that large.