Implement research that generates Keyword+Synonyms structure including morphology

[nataliashitova]

We need to implement a research that generates Keyword+Synonyms structure to be used in all keyword-based assessments. For languages that already have morphological support the structure should include word forms.

{
      keyphraseForms: [
              // forms of every word from the keyphrase
             [ form1, form2, ... ],  // 1st content word from the keyphrase
             [ form1, form2, ... ],  // 2nd content word from the keyphrase
             ...
      ],
      synonymsForms: [
             [  // forms of every word from the 1st synonym
                   [ form1, form2, ... ],  // 1st content word from the 1st synonym
                   [ form1, form2, ... ],  // 2nd content word from the 1st synonym
                   ...
             ],
             [  // forms of every word from the 2nd synonym
                   [ form1, form2, ... ],  // 1st content word from the 2nd synonym
                   [ form1, form2, ... ],  // 2nd content word from the 2nd synonym
                   ...
             ],
             ...
      ],
}

For Free and for all languages that do not have morphological support the structure should be the same, but without synonyms and / or word forms.

{
      keyphraseForms: [
              // every word from the keyphrase
             [ word1 ],  // 1st word from the keyphrase
             [ word2 ],  // 2nd word from the keyphrase
             ...
      ],
      synonymsForms: [
             [  // every word from the 1st synonym
                   [ word1 ],  // 1st word from the 1st synonym
                   [ word2 ],  // 2nd word from the 1st synonym
                   ...
             ],
             [  // every word from the 2nd synonym
                   [ word1 ],  // 1st word from the 2nd synonym
                   [ word2 ],  // 2nd word from the 2nd synonym
                   ...
             ],
             ...
      ],
}

Future adjustments of assessments that include morphology are described in #1558.

For Free and Premium the function words should be removed from the arrays of word forms, if the respective lists are available.