Standardize and update rationales

joke -> joke or hoax; link What Wikipedia is not; encyclopedic tone ->
formal, neutral encyclopedic tone, rmv excess space
Clarify difference between `blp` and `van`

van is for pages that should be blanked and speedied, but blp is
essentially "AGF, blank, and ask for sources"
Work on #8 (csd and blanking) some more

Only display `csd` if the rationale is vandalism or copyvio, per
[[Template:AfC submission/comments]]; just enable blanking for `blp`;
and otherwise hide the CSD option