recommendation framework must ignore system tags

[Robert Jäschke]

The recommendation framework normalizes tags (e.g., removes symbols, etc.) This causes system tags like sys:relevantfor:GROUP to be transformed to sysrelevantforGROUP and thus to break. All tags starting with "sys:" should either be ignored or not normalized.

[Daniel Zoller]

added flag to remove system tags from tag recommendation

fixes #2576

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