fix: assertAdmin sending wrong ability name to gate

[clarkwinkelmann]

Fixes #0000

Changes proposed in this pull request:
Currently, the full user record serialized as JSON is being sent to the gate when checking for admin privilege, while the intended ability name administrate is sent as the gate parameter as string. This works because the gate will always return true for admins, but this means extensions can't customize the administrate ability since it's never actually passed.

I have not tested the change locally, but I wanted to start with the PR right away since it's easier to explain this way instead of creating a whole issue about it.

This is the output from my gate logger extension prior to the fix, it's supposed to say Gate Logger administrate(Actor: Admin, [GLOBAL])

[2022-07-03 21:24:12] flarum.DEBUG: Gate Logger {"id":1,"username":"Admin","email":"admin@http418.ch","is_email_confirmed":1,"password":"$2y$10$lLDDVBiags5q80XY8imjQuc\/Bk6EBGd.CkEvfoKZI.oxPMyCtsfFy","avatar_url":"http:\/\/1.3.flarum.localhost\/assets\/avatars\/predefinedAvatar:RFjlw9i7awEUwtzz.png","preferences":{"notify_discussionRenamed_alert":true,"notify_selectBestAnswer_alert":true,"notify_selectBestAnswer_email":true,"notify_awardedBestAnswer_alert":true,"notify_awardedBestAnswer_email":false,"notify_bestAnswerInDiscussion_alert":false,"notify_bestAnswerInDiscussion_email":false,"notify_postMentioned_alert":true,"notify_postMentioned_email":false,"notify_userMentioned_alert":true,"notify_userMentioned_email":false,"notify_discussionLocked_alert":true,"notify_postLiked_alert":true,"discloseOnline":true,"indexProfile":true,"locale":null},"joined_at":"2022-05-17T17:49:12.000000Z","last_seen_at":"2022-07-03T21:24:12.000000Z","marked_all_as_read_at":null,"read_notifications_at":null,"discussion_count":6,"comment_count":9,"read_flags_at":null,"suspended_until":null,"suspend_reason":null,"suspend_message":null,"best_answer_count":0,"social_buttons":"[{\"title\":\"Test\",\"url\":\"https:\/\/extiverse.com\/\",\"icon\":\"favicon\",\"favicon\":\"data:image\/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjxzY3JpcHQ+YWxlcnQoJ1hTUycpPC9zY3JpcHQ+PC9zdmc+Cg==\"}]"}(Actor: Admin, administrate::class)
Decision: ALLOW (Admin role)  

Maybe this is worth an integration test? I'm not sure which feature could be tested for administrate extensibility though.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)