Webgrid

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- Add optional i18n support using morphi (3627e8f_)
NOTE: there is a slight change that could result in a behavioral change during
upgrade! Please see the 'Upgrading' section in the readme for more
information!

.. _3627e8f: https://github.com/level12/webgrid/commit/3627e8f

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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