Transliteration: smarter "retroactive" transliteration of existing file names.

The setting to transliterate file names (admin/config/media/file-system) is on by default. A good thing IMO, but the help text needs some love, plus we can be smarter than that. So...

1. The help text currently reads:

   If you enable this setting later, you may want to retroactively transliterate existing file names.

   ..."later"?? ...later than what? Also "retroactively"?? How many non-English speaking users even know what this means? I think that it would be better worded like so:

   If you enable this setting after files have been uploaded to your site, you may want to transliterate existing file names.

2. As for the "smarter" part: how about we hide this part of the help text entirely? ..then we can have a check run at cron or something (only if "Transliterate file names during upload" is enabled) and then display a site status report message like this:

   There are currently files uploaded to your site that contain non-ASCII characters in their file names. You may want to transliterate them.

   This warning can be merged with the existing "File system" section of the site status report.

   Now, once this check has been done (only if "Transliterate file names during upload" is enabled) and there are files with non-ASCII characters, we can (additionally to the site status page) also update the help text of this setting by appending the "There are currently files uploaded..." text.