Issue #1570848: Fixed remote stream wrapper imports failing with error: Column 'field_*_display' cannot be null.

Hardcoding permissions is a problem, because there are differences between
Media 2.x and Media 1.x.

 - Add trailing spaces to sentences in the changelog.
 - Use elseif instead of else if.

 #1386388: Fixed Failed to get the file object for temporary://FILENAME when importing from Mailhandler.

This were two steps:

 - Allow import by file URI (see issue #1386410).

 - Make a permanent copy of temporary:// files.

   First select one of the allowed schemes. Try public, then private, then any
   other scheme.
   Then try to copy the temporary file, making it permanent on the way.

Before that an awful workaround had to be used. Issue #1121808 is fixed, so
this is no longer nescessary.

Works together with [#860748] (Config for mappers?). Allow the user to
define in the mapping ui, that the source has no file extensions and
should match regardless.

Add MediaFeedsLibraryProvider that allows to map existing files by their
filename.

To allow more Media Feeds level providers factor out a base class. Let
the existing provider extend that class.