Webgrid

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- XLSX formats are cached for performance

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- Add test helper `assert_rendered_xls_matches` in `webgrid.testing`
- Add support for XLSX, and CSV renderers
- If you have xlsxwriter installed, xlsx export link will appear
- DEPRECATED old export mechanism
- If you are calling `g.xls.as_response()` please replace that with
`g.export_as_response()` which will select the correct renderer and return
the response correctly
- If xlsx is enabled you will need to make this above change to enable xlsx exporting

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- fix CSS collision in tr classes with Bootstrap
- change multiselect to use body as the container for the multiselect list

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- session_override GET arg added to allow patching additional operators into the session (rather than overriding session filters)

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- limit XLS sheet names to 30 characters, per the Excel format limit

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- corrected the results of Filter.is_active to account for default operation with no value
- fixed formencode requirement for python 3
- update options filter error to include class name