Add unique key

[alexalouit]

unique key for mysql unique key for sqlite

[alexalouit]

we need to add unique key for pgsql

[jtojnar]

Do not we also need to handle the duplicates before adding the indexes?

[jtojnar]

Rebased this onto master and resolved conflicts.

But yeah, this still needs to handle past duplicates otherwise it will fail with PDOStatement: UNIQUE constraint failed: items.uid, items.source