Mark activity read by ids, all, a category, or a subject

Send exactly one selector: ids, all, category, chat_id, link, or subject_type with subject_id. By subject marks every row about that subject read, wherever it was counted -- opening a post, job or grant clears everything said about it. By chat marks every row that links into that chat (mentions, group adds, missed calls), the selector a chat screen uses. By link marks every row whose in-app link is that address (or that address with a query string), e.g. /acting/<id>: the selector for a screen whose rows have other subjects (an approval's subject is the exercise, the screen the mandate). A row that is read counts as seen too. 422 no_selector when none is sent.