wxr importer refactoring and improvements

[tellyworth]

This patch includes several improvements to the wxr importer:

• The code for handling author names and mapping them to local user names is refactored and simplified. The old code was copied straight from the MT importer; the new version is easier to follow and runs faster on sites with large user tables.

• Gzipped files are supported if zlib is available (this fixes one half of #5485).

• There's a simple check to confirm whether or not the file appears to be the correct format. An error is displayed if it is not. Previously the importer would silently ignore a file if it was the wrong format.

• Filters are added to make it possible to turn off the ability to add new users and download files. There's also a filter for specifying a maximum size for attachment imports. These are mainly of interest for wpmu.

• Failed attachment downloads are removed.

• The uploaded import file is given a '.import' extension.

• import_start and import_end actions are added.

(In [6477]) Importer refactoring from tellyworth. fixes #5522