All existing link will keep their permalinks.
New links will have smallhash generated with date+id.
The purpose of this is to avoid collision between links due to their creation date.
* search type now carried by LinkDB in order to factorize code between different search sources.
* LinkDB->filter split in 3 method: filterSearch, filterHash, filterDay (we know what type of filter is needed).
* filterHash now throw a LinkNotFoundException if it doesn't exist: internal implementation choice, still displays a 404.
* Smallhash regex has been rewritten.
* Unit tests update