Creating tags for BNHA's official AUs

BUR #17247 has been approved by @nonamethanks.

mass update favgroup:21841 -> 2nd_popularity_poll_(boku_no_hero_academia)
mass update favgroup:21840 -> 7th_popularity_poll_(boku_no_hero_academia)

Two of the most popular official AUs created for BNHA. The first one could alternately be called Datte Atashi no Hero (Boku no Hero Academia)

The bulk update request #17247 (forum #243170) has been approved by @nonamethanks.

BUR #48581 has been approved by @evazion.

create alias 2nd_popularity_poll_(boku_no_hero_academia) -> boku_no_hero_academia_2nd_popularity_poll
rename 6th_popularity_poll_(boku_no_hero_academia) -> boku_no_hero_academia_6th_popularity_poll
rename 7th_popularity_poll_(boku_no_hero_academia) -> boku_no_hero_academia_7th_popularity_poll
rename 8th_popularity_poll_(boku_no_hero_academia) -> boku_no_hero_academia_8th_popularity_poll
rename 9th_popularity_poll_(boku_no_hero_academia) -> boku_no_hero_academia_9th_popularity_poll

These don't need to be qualified with the copyright name and just make it confusing for any future popularity poll tags from other copyrights. Having it the other way around and not qualified makes more sense to me.

I am prone to badly formatting BURs I'm not used to making so feel free to point and laugh if I made a mistake (and tell me where so I can fix it)

The bulk update request #48581 (forum #390459) has been approved by @evazion.

These tags are kind of confusing in that they're basically alternate costumes taken from the popularity poll promotional art, compared to the Touhou popularity polls which are for reactions to the results of the polls.