Add optional "tag" to a member's functionary post

[KingJohanna]

Some functionary posts have special responsibilites and are therefore granted extra door-accesses; athlete responsible for racket sports, for example (read https://fsektionen.se/dokument/504).

Introduce a sort of tag to a functionary post, so that the right people are granted their special door-accesses automatically (see https://fsektionen.se/admin/dorrar).