Add support for parsing datetime strings and for querying on a resource's "modified/last update" time

• Add support for parsing datetime strings in addition to date strings for date/time queries. Requires that a field be registered with Orthanc and that the fields for a resource be indexed.
• Added Modified and LastUpdate to the fields which can be queried, allowing for results to be filtered to those modified in a specific window.
• Added Modified and LastUpdate to the fields for ordering results.
• Fixed "ghost bug" which was causing resources to be left in the cache after they had been removed from the database (#29 (closed)).
• Added model and API endpoint for series "comments".
• Added validation module
  • OrthancBaseForm: class based on pydantic to be used for data validation and cleanin
    • clean: perform data conversion and field validation. Based on Django form field of the same name.
    • save: persist data from the form to a model instance