ech00147 export can't deal with keywords with leading/trailing whitespace

[deiferni]

https://sentry.4teamwork.ch/sentry/onegov-gever/issues/63411/

steps to reproduce:

• Create a keyword with trailing whitespace e.g. via PATCH request with the following body to a document or dossier:

{
  "keywords": [
    {
      "title": "niuu ",
      "token": "niuu "
    }
  ]
}

• run an ech00147 export for the dossier. the export will fail with the following error:

SimpleTypeValueError: Type {http://www.w3.org/2001/XMLSchema}token cannot be created from {http://www.w3.org/2001/XMLSchema}token: SCHLAGWORT: 20016_TEST_WEHRLE_001 
(21 additional frame(s) were not displayed)
...
  File "pyxb/binding/basis.py", line 943, in __init__
    self.xsdConstraintsOK(location)
  File "pyxb/binding/basis.py", line 1069, in xsdConstraintsOK
    return self.XsdConstraintsOK(self, location)
  File "pyxb/binding/basis.py", line 1035, in XsdConstraintsOK
    value = cls._XsdConstraintsPreCheck_vb(value)
  File "pyxb/binding/datatypes.py", line 1061, in _XsdConstraintsPreCheck_vb
    if not cls._ValidateString_va(value):
  File "pyxb/binding/datatypes.py", line 1086, in _ValidateString_va
    raise SimpleTypeValueError(cls, value)

SimpleTypeValueError: Type {http://www.w3.org/2001/XMLSchema}token cannot be created from {http://www.w3.org/2001/XMLSchema}token: SCHLAGWORT: 20016_TEST_WEHRLE_001 

We should either fix the ech00147 export or disallow creating keywords with trailing whitespace. Also as this seems to have been in production for a while we must provide a cleanup upgrade to remove keywords with trailing/leading whitespace and potentially merge keywords.

See also #6312 for the sibling issue describing the restapi input sanitation we should discuss. If we fix the api, this issue here can be closed.