Add search on tags

[MrPetovan]

Suggested by @tobiasd :

If you follow a link in the sidebar, say "friendica" I'd expect seeing the people having tagged themselves with "friendica" insteat you end up on the search for friendica which includes node URLs ala "friendica.example.com".

Both the trending topics aside and the profile hashtags trigger a regular search on all the profile fields, which isn't expected. This new search type would only look into tags.

[MrPetovan]

During my work on the next version of the directory I tried to add a tag-only search and it was rather disappointing. Maybe present the exact tag results before the general fulltext search results?

[MrPetovan]

Moved to friendica/friendica-directory#9