(bug 52387) Introduction of FetchedEntityContentSerializer

Moved the code for creating special serialization as required by the front-end (deserialized in
wb.serialization.FetchedContentUnserializer) in its own class. This is based on the current
serialization system which has some design issues.

The whole concept of how we get these informations to the front-end should be redesigned at some
point, for now this is a change for saving some code when requiring the same logic on special
pages where our front-end widgets requiring this information should be used. This also adds some
basic test to previously untested functionality.

Change-Id: I6d53bdda60297f79cec192d19f3ca1534e652284
Merge "(bug 52387) Introduction of FetchedEntityContentSerializer"