Unify handling of different datatypes

There should be some kind of class / function to do all basic datatype handling in the system:

 - BLOB datatype will alwalys be displayed by a download link (and not the content of the column)
 - NULL should always get some special formatting
 - If "showFullColumnContents" is turned off, all kinds of column values should be limited to X chars

All these function are currently implemented in different controller actions or templates and should be unified to one simple class.