added simplistic UI for tagging pages

opted to go with comma-delimited tags, as that's simpler and thus more
user-friendly than TiddlyWiki's bracketed serialization (as provided by
`tiddlyweb.model.tiddler:tags_list_to_string`) - of course that means
commas are not allowed in tags, but that seems sensible anyway

of course the UI itself is not very user-friendly yet

/cc @cdent @pads