Tag unions and intersections

[ryan]

As was done for categories in #4750, let's add private tag__in, tag__and, and tag__not_in query vars.

Let's also decide on how to specify intersections and unions in the public query vars. We have tag_id and tag. tag_id currently takes a single numeric ID, and tag takes a single tag slug. tag is the var we use when creating links and such. tag_id isn't exposed anywhere at the moment. I think we can extend tag to allow the use of "+" to request tag intersections and commas to request unions. Only one or the other can be specified in the same request.

tag+tag2+tag3

tag,tag2,tag3

[ryan]

(In [6011]) tagin, tagnot_in, and tag_and queries. fixes #4895

[ryan]

(In [6012]) tag_id not tagid. see #4895

[ryan]

Patch parses "tag" queries that can plus and comma. Intersections are parsed into tag_slug__and and unions are parsed into tag_slug__in. The joins are rather hairy. Don't know if it would be best to get the IDs for the slugs in a separate query to spare the extra join.

[markjaquith]

Regarding tag_id, I don't think we should ever expose that because I don't think we should ever rely on that ID being static.

[ryan]

It's kinda there mainly for plugins that already have an ID. We could keep it and make it private.

[ryan]

(In [6021]) Tag unions and intersections. fixes #4895