Properly handle uploaded files for blogroll imports

[apokalyptik]

Fix 1 since wp_remote_fopen does not properly handle local files versus remote urls (and by the name of the function probably should not do so anyhow ("remote" and all that) Uploading a file instead of linking to a file already on the web for import caused $opml to be set as boolean false. This in turn was not very parse-able by the xml routines. So I wrapped the setting of $opml in an if statement that would use file_get_contents() for local files and wp_remote_fopen() for remote files.

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Fix 2 unlinking an uploaded file for import should pass the file through the wp_delete_file filter.

(In [5438]) Handle local and remote blogroll imports. Props apokalyptik. fixes #4243 for 2.2

(In [5439]) Handle local and remote blogroll imports. Props apokalyptik. fixes #4243 for 2.3