Refactored invoice system

[kevinpapst]

This PR refactored the invoice process:

• adds directory scan for custom templates
• gets rid of all configuration values and auto-detects them
• allows different renderer for custom output formats

TODO:

• basic refactoring
• auto detect number generator / renderer / calculator
• basic preview template to display all entries that will be included in the invoice document
• invoice renderer
  • new renderer interface for custom invoice document types
  • twig renderer for default templates
  • renderer for CSV (fixes Feature Request - Excel Output of times #221)
  • renderer for Excel (fixes Feature Request - Excel Output of times #221)
  • renderer for Word (fixes Add support for MS Office templates #283)
  • renderer for Open Office (fixes Add support for OpenOffice templates #223)
• translations
  • new renderer
  • new user calculator
• documentation
  • config update ( in release notes)
  • invoice directories scan
  • custom invoice templates
• unit tests
• integration tests

[lock]

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