Remove shown content from current randomization possibilities.

[RasmusKard]

Don't suggest same content again.

Save suggested content IDs in cookies? (Re-randomization when hitting already suggested content ID can be costly when sample space is large but is outweighed by the chance of hitting same ID being low with large sample space.)

[RasmusKard]

Completed in a9ba33e81fffdd328c8d2ef7e051e8f1dbf7d51e