prepare to support maintainer actions

[tayloraswift]

the next big priority for us should be supporting maintainer configuration through the web UI, since Swift Package Index’s experience with checked-in configuration files has not been great.

there will be some actions that are initially admin-only, and some actions available to maintainers.

• Uplink, Unlink, Delete these have major potential to disrupt the site’s operations, so they should remain admin-only.
• Hide/Unhide package ditto.
• Rename package this should be okay
• Transfer package realm this should be admin only for now, otherwise people could add things to realms they do not own
• Create package realm should be admin-only, users should have realms created for them
• Alias package should be admin-only, as it consumes names from the global namespace
• Index package tag could be abused to DoS the site, so it has to be admin-only for now, but we should place a rate limit on it as it is a common task that maintainers would want to do. we should consider providing an alternative Queue package for tags fetch instead.

[tayloraswift]

tasks:

• [x] Refactor Swiftinit.TagsTable to have three permissions levels instead of two. Hide admin-only actions from the first two levels.
• [x] Show Package ID (not name) under Package Settings
• [x] For maintainers, show when the package is scheduled to next have its tags fetched
• [x] For maintainers, expose a function that queues a package for tags refetch.
• [ ] Set up server plugin that periodically visits packages and regenerates missing alias records, which is needed to complete #102
• [ ] Set up server plugin that periodically visits packages and checks if their owners are registered in the accounts DB, which is needed to complete #111