Invoicegenerator

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- Fix conf relative import
- Use python native function splitlines for notes

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- Add property number to object Invoice
- Replaced variable symbol for invoice number in invoice header
- Update Czech translations

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- Bug fix previous commit

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- Create proforma invoice

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- Change date format for qr code generator
- Disable converting datetime to string on Invoice
- Disable rendering empty values

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- The displayed number of pages only when there is more than one
- Rename Date to Date of exposure
- Use pillow instead of PIL