3782: keep log viewer code in _show_log.html.erb

3782: keep log viewer code in _show_log.html.erb

Keep log-related restrictions in the log viewer and not in general
purpose workbench code:

* Add Rails.configuration.log_viewer_max_bytes to specify the maximum
  number of log bytes to display
* The log viewer specifies a byte limit via the HTTP Range header. The
  Content-Length of the response is set to the minimum of the 'size'
  query parameter and the byte range. If the log appears to have been
  truncated, the viewer reports it in the 'log_viewer_overview' pane.
* CollectionsController.FileStreamer either delivers the entire file
  or exactly opts[:maxbytes] bytes, whichever comes first.  It does not
  add any log-specific message.