feat: advanced admin registry extenders

[SychO9]

Addresses #4200 (comment)

Changes proposed in this pull request:

• Allows changing an existing admin setting/permission.
• Allows changing the priority of an existing admin setting/permission.
• Allows removing an existing admin setting/priority.

The current caveat is that when extension A wants to do any of this to a setting/permission from extension B, it has to specify the extension ID in the constructor of the Admin extender, and it also has to be booted after extension B.

Examples

// current extension: nicknames

new Extend.Admin('flarum-sticky').replaceSetting('flarum-sticky.only_sticky_unread_discussions', (original) => ({
  ...original,
  label: 'test',
 })),

new Extend.Admin('flarum-sticky').setSettingPriority('flarum-sticky.only_sticky_unread_discussions', 500),

new Extend.Admin('flarum-sticky').removeSetting('flarum-sticky.only_sticky_unread_discussions'),

new Extend.Admin('flarum-sticky').replacePermission(
  'discussion.sticky',
  (original) => ({
    ...original,
    icon: 'fas fa-at',
  }),
  'moderate'
),

new Extend.Admin('flarum-sticky').setPermissionPriority('discussion.sticky', 'moderate', 500),

new Extend.Admin('flarum-sticky').removePermission('discussion.sticky', 'moderate'),

new Extend.Admin('core-basics').removeSetting('welcome_title'),