Move all different implementation of 'getValidAssignmentList' into
  Utils.py

    - getValidAssignmentList: Used everywhere, no cache, accepts
    start_date only setting.

    - getCachedValidAssignmentList: Same as above, however cached over
      the transaction per user (parameter)

    - hasValidAssignment: Return True as soon it finds the first
      valid one, with no cache.

  Allow import *ValidAssignmentList directly on Restricted Python so we
  don't need to write External Methods to re-use the implementation.

  Related code was updated accross the business templates, the solo
  difference in implementation is that StandardSecurity use
  transactional cache version rather then no cache. This prevents code
  re-calculate the assignment list more then once accross the
  transaction. Due this cache was updated acordinally.

  Other locations were kept using uncached version for compatibility
  since it has low impact or it is not widely used code.

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