marshmallow_utils: include DynamicSanitizedHTML

[Samk13]

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Description

closes #190

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[ntarocco]

If possible, I would avoid adding a dependency on invenio-pages to invenio-administration.
I think that we should change the code: the part custom_mapping[field_type.inner.__class__]}, should be smarter and first check of the type, then check for the parent classes (to traverse up the inheritance). If not found, than fail.
In this way, it will go up to the parent class and find SanitizedHTML.

WDYT?

[Samk13]

Done. I implement find_type_in_mapping that locates a specific field type within a given mapping (that is passed as an arg as well) by iterating through its inheritance chain so it will search for the field type in a custom mapping dictionary and returns the corresponding mapped value. If the field type is not found, it raises a KeyError.
Not sure if we should catch the keyError somewhere else?
I created some tests as well. Let me know if you have suggestions for further improvements.