[feature] Handle retractions of domain permission subscription entries (#4261)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

This pull request adds logic for nicely handling retractions of entries from domain permission subscriptions.

See docs for how this works but basically retracted entries will either be removed (and possibly picked up by a lower-prio subscription), or orphaned (and then possibly adopted), depending on the config of the domain permission subscription.

closes https://codeberg.org/superseriousbusiness/gotosocial/issues/4101

## Checklist

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If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [x] I/we have added tests that cover new code.
- [ ] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4261
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

internal/api/model/domain.go

@@ -131,6 +131,9 @@ type DomainPermissionSubscription struct {
 	// If true, this domain permission subscription will "adopt" domain permissions which already exist on the instance, and which meet the following conditions: 1) they have no subscription ID (ie., they're "orphaned") and 2) they are present in the subscribed list. Such orphaned domain permissions will be given this subscription's subscription ID value.
 	// example: false
 	AdoptOrphans bool `json:"adopt_orphans"`
+	// If true, then when a list is processed, if the list does *not* contain entries that it *did* contain previously, ie., retracted entries, then domain permissions corresponding to those entries will be removed. If false, they will just be orphaned instead.
+	// example: true
+	RemoveRetracted bool `json:"remove_retracted"`
 	// Time at which the subscription was created (ISO 8601 Datetime).
 	// example: 2021-07-30T09:20:25+00:00
 	CreatedAt string `json:"created_at"`
@@ -198,6 +201,10 @@ type DomainPermissionSubscriptionRequest struct {
 	//	in the subscribed list. Such orphaned domain permissions will be given this
 	//	subscription's subscription ID value and be managed by this subscription.
 	AdoptOrphans *bool `form:"adopt_orphans" json:"adopt_orphans"`
+	// If true, then when a list is processed, if the list does *not* contain entries that
+	// it *did* contain previously, ie., retracted entries, then domain permissions
+	// corresponding to those entries will be removed. If false, they will just be orphaned instead.
+	RemoveRetracted *bool `form:"remove_retracted" json:"remove_retracted"`
 	// (Optional) username to set for basic auth when doing a fetch of URI.
 	// example: admin123
 	FetchUsername *string `form:"fetch_username" json:"fetch_username"`