Merge pull request #457 from NicoleRauch/master

Add Sympa cache invalidation - fixing Bug #453
Added optional limit to the resources, added loading of resources with limits from DB.
We should rather update the activity that is stored under the previousUrl! Added tests to verify that DB is ok with null in query.
Merge commit '6ab394614db78682e0363afde6d1bd08b0dbbc5a'
First experiments towards lazy conversion of database objects: Activity now contains a database object.