Lizard-map

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- Fix OpenStreetMap pink tiles at Firefox.

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- Flot graphs: fallback to .axes label if one is available, because flot only supports a single ylabel.

- Flot graphs: pass x and y limits so we can determine tick size.

- Multiple select: don't show animation when nothing is found.

- Mapnik WMS rendering: reduce memory usage because of buffers being copied multiple times.

- Changed lots of core stuff: no longer combine workspace layers into a single WMS layer.

- Added multi-url legend support.

- Fix some bad hover_popup code.

- OpenLayers: fix iPad.

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- If a legend_image url is empty, we don't show the legend anymore.

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- In a map view, you can now provide ``.extra_wms_layers()`` to add extra
WMS to the map. Handy for layers that are really part of a specific content
item. The list of dictionaries that this method has to return is really an
unfriendly API: this needs refactoring later on.

- Internal refactoring. Renamed ``.maps()`` to ``.backgrounds()`` in the
views. This (hopefully) isn't used externally.

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- Importing JSONField in fields.py as otherwise the migrations fail.

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- Requiring newer django-jsonfield version (which works with django's multi-db
functionality). Removed our custom JSONField in favour of
django-jsonfield's one.