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.
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