Clean up VPOExport and friends.

[Jeroen T. Vermeulen]

Now that the POExport view is gone, it's becoming obvious that the vpoexport module is basically dead weight.

Some things we can clean up with relative ease:
 • VPOExportSet's get_pofile_rows and get_pofile_changed_rows belong in POFile (with dummy implementations in DummyPOFile).
 • A lot of elements in VPOExport are unnecessary: potemplate, template_header, language, variant, translation_file_comment, translation_header, is_translation_header_fuzzy.
 • VPOExport can take msgid_singular and msgid_plural from the POTMsgSet which is fetched anyway.
 • With those gone, there's no need to keep POTMsgSet and its msgids in the join that fetches a POFile's translation rows.

More cleanups will become obvious after this; see bug 401598. The export script creates about 25M VPOExport objects when generating a full Ubuntu language pack, so eliminating just one reference from VPOExport should save at least 200 MB of data traffic on a 64-bit server (plus dict overhead).

[Jeroen T. Vermeulen (jtv)]

Weekend branch.

[Matthew Revell (matthew.revell)]

Retargeting to 3.0, which is the new name for 2.2.9. Haven't renamed the milestone as the 3.0 milestone is in a different series.

[Jeroen T. Vermeulen (jtv)]

Fixed in devel 9006