Mark all tag <input>s with a `data-autocomplete` attribute, instead of
hardcoding a list of html IDs to autocomplete in javascript.
This way should be less error prone. It fixes autocomplete in several places:
* Autocomplete for the search box on /posts didn't work in the
responsive layout. This was because /posts has two search boxes that
both have the id `tags`: one in the normal sidebar, and one in the
responsive tag list. $("#tags") only initialized autocomplete on the
first one.
* Autocomplete didn't work on the aliases or implications pages. This
was due to selecting the wrong html ids.
Fixes display of borders on the previews and such. Also fix the yellow
duplicate background not always working because Post#has_dup_tag? didn't
return a boolean.
