Django-odk

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* rm modified_on & modified_by field
* add convert xls 2 xml (use of pxforms)
* new odkdata app with 2 methods: create_model (use of xlsconv) & load_submit_data
* add datetime fields in XForm & XFormSubmit to check data convert & data load_submit_data
* modify view for Odk load with 4 steps (cfr. README file)

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* add md5: string in <hash> tag of xformsList.xml

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* rm xform from html
* change input type submit to button type

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* rm xform FK from submit model

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* Adapt locale fr
* Launch migration in EN

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* Add locale fr
* Remove AVAILABLE_TXT and SUBMITTED_TXT from settings
* Replace self.headers by self.setdefault in odk.openrosa.view_pull