Add support for hosting providers that disable readfile()

[Deltik]

Issue came from Gitter, where @wonozhong had this issue:

There are two usages of readfile() in e107 that can be abstracted to use an alternative to readfile():

[CaMer0n]

Hoping this change is unrelated to #3520

[Deltik]

@CaMer0n: #3520 looks unrelated.

In that issue, the user referenced this thumbnail URL:

http://www.outlawsracing.com/thumb.php?src=e_MEDIA_IMAGE%2F2018-10%2Fe-packchampweb.png&w=800&h=0

That translates to this source URL:

http://www.outlawsracing.com/e107_images/2018-10/e-packchampweb.png

At the time of writing, that URL returned HTTP 404, which explains the by-design behavior of thumb.php redirecting to https://placehold.it/800x0.

[CaMer0n]

Thank you @Deltik !