Rename obsolete tag from "obs" to disambiguate from "obsc"

[opencooper]

Currently, the tag for an obsolete term is "obs", and for an obscure term, "obsc". I think the fact that "obs" could stand for either leads to unnecessary confusion.[0] A small change to lessen the potential for confusion would be to add an extra letter to "obs", e.g. "obsl".

[0]: A recent example, but I've also wondered in the past if an editor meant to apply the other tag.

[robinjmdict]

I'd be in favour of reviving the "rare" tag (six entries still have it, funnily enough) and using that instead of "obsc". It's impossible to confuse with "obs" and I prefer "rare" to "obscure" as an adjective for describing uncommon terms. It's also what Oxford uses.

[Marcusjmdict]

Sounds good to me, though if we go by intuitive meanings of the words "rare" would obviously apply to a whole lot more words than "obscure" does. Not necessarily a bad thing, though.

[JMdictProject]

I quite forget why "rare" was deprecated or fell into disuse. I'd hate to have to explain the precise difference as applied to entries. I think changing "obsc" to "rare" is not a bad thing. It's actually a relatively simple thing to do as the actual tag in the DB is numeric and the text/number pairs are in a table. I may as well go and change the existing 6 "rare" entries to "obsc" (pro tem) as preparation. There may be some other tag changes.

[JMdictProject]

Now being handled in #68