Graceful handling of image filetypes not supported by SimpleImage

[ehawman-rosenberg]

Closes #1048

Currently, if a file returns TRUE for isImage() (which checks vs mimetype), but isn't supported by SimpleImage, we get a broken link
.

SimpleImage supports "gif", "jpg", "jpeg", "png", "webp", and "bmp" formats (also "jfif" which is just another jpg variant). We also have a way to handle svg file previews.

As such, I am validating whether an image is embeddable vs these extensions, and I've changed the structure of the preview generation in manage-files.php and template.php to generate the graceful /thumbnail-unavailable.png

Alternatively, we could keep isImage filtering by mimetype and then have an exception from SimpleImage's failure to generate a thumbnail trigger thumbnail-unavailable. This is probably the cleaner way but I don't have time :)

[ehawman-rosenberg]

I went to make some other changes and realized I was forking off of an edited Develop branch (oops). I'm going to close this pull and reopen once I've sorted all my conflicts out.