Lizard-map

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- Changed empty workspace text.

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- Raising workspaceitemerror when the json we get isn't decodable. More
robust code this way.

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- Made popups more consistent (shadow color and size).

- Added 'continue to site' link in introductory video popup.

- Showing reload link in the statistics part that is shown when the date range
has been changed.

- Adding extra safety measure so lizard-map doesn't crash on faulty
shapefiles.

- Fixed collage graph editing: made a check more robust for missing parameters.

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- Re-enabled custom y tick locator amount selection.

- Showing the table now looks at the table belonging to the button by means of
a wrapper div, which is more reliable.

- Fixed the problem that a hidden popup would remain populated and re-evaluated
(ticket 2892).

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- Added bare try/except around wms workspace layer looping to prevent
the function to crash.

- Added transparency_slider to context variables. From now on the
transparency_slider is enabled by default.

- Added optional popup_video_url parameter to the ``homepage()`` view. This
video url, when used, should point to the embed urls of YouTube videos.
Currently the sizes are hardcoded, so this is work-in-progress.
The video is shown once per day, max.

- Modified graph: less yticks, bigger ylabel, dutch monthnames,
yearlabel is now at first tick of year.

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- Fixed tab setup in graph popup: no more unneeded reloading of the html. Also
the graphs aren't reloaded more often than necessary.

- Added force_legend_below parameter to ``legend()`` to force the legend to be
placed below the graph.

- Added function to create a image from reguest and response to save
it as .png on client side (views.py, urls.py, lizard_wms.js)

- Added declaration to OpenLayers' ImgPath to use our own dark theme location.

- Implemented tipsy in some places.

- Worked on appearance of graph-popup.