toaster: tweaks to recipe file downloads

toaster: tweaks to recipe file downloads

* In the custom images table, show the recipe file and download icon
only when the recipe file exists and can be downloaded.

* Also in the custom images table, make sure the download icon tooltip
shows on hover.

* In the custom image details page, show the correct icon (download)
next to the recipe file in the right hand column.

* In the custom image details page, show the recipe file and download
icon in the right hand column only when the recipe file exists and can
be downloaded

* Also in the custom image details page, simplify the help text we show
when the 'download' button is disabled: it gave so much information about
what's actually happening under the hood that it was a bit hard to
follow.

Signed-off-by: Belen Barros Pena <belen.barros.pena@linux.intel.com>
Signed-off-by: Elliot Smith <elliot.smith@intel.com>