Fixes#784
From now, searching for tags with an empty value will return only not tagged links,
with the search bar showing `x results [not tagged]`.
Note that using the api, the searchtags request parameter must be set to `false` to get the same result.
- [ ] Update API doc
* 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
* Searching '-mytag' will now exlude all shaares with 'mytag' tag.
* All tags starting with a '-' are renamed without it (through the Updater).
* Unit tests.
Minor code changes:
* LinkDB->filter() can now take no parameters (get all link depending on logged status).
* tagsStrToArray() is now static and filters blank tags.
* introduce class LinkFilter to handle link filter operation (and lighten LinkDB).
* handle 'private only' in filtering.
* update template to prefill search fields with current search terms.
* coding style.
* unit test (mostly move from LinkDB to LinkFilter).
PS: preparation for #358#315 and 'AND' search.
