Added MailgunEmailSuppressionList

refs https://github.com/TryGhost/Team/issues/2253
refs https://github.com/TryGhost/Team/issues/2254
refs https://github.com/TryGhost/Team/issues/2255

This is the implementation of the EmailSuppressionList specific to
Mailgun, it listens for the new events and stores suppressions based
on those.

ghost/core/core/server/services/email-suppression-list/MailgunEmailSuppressionList.js

@@ -0,0 +1,117 @@
+const {AbstractEmailSuppressionList, EmailSuppressionData} = require('@tryghost/email-suppression-list');
+const {SpamComplaintEvent, EmailBouncedEvent} = require('@tryghost/email-events');
+const DomainEvents = require('@tryghost/domain-events');
+const logging = require('@tryghost/logging');
+
+/**
+ * @typedef {object} IMailgunAPIClient
+ * @prop {(email: string) => Promise<any>} removeBounce
+ * @prop {(email: string) => Promise<any>} removeComplaint
+ * @prop {(email: string) => Promise<any>} removeUnsubscribe
+ */
+
+class MailgunEmailSuppressionList extends AbstractEmailSuppressionList {
+    /**
+     * @param {object} deps
+     * @param {import('bookshelf').Model} deps.Suppression
+     * @param {IMailgunAPIClient} deps.apiClient
+     */
+    constructor(deps) {
+        super();
+        this.Suppression = deps.Suppression;
+        this.apiClient = deps.apiClient;
+    }
+
+    async removeEmail(email) {
+        try {
+            await this.apiClient.removeBounce(email);
+            await this.apiClient.removeComplaint(email);
+            await this.apiClient.removeUnsubscribe(email);
+        } catch (err) {
+            logging.error(err);
+            return false;
+        }
+
+        try {
+            await this.Suppression.destroy({
+                destroyBy: {
+                    email_address: email
+                }
+            });
+        } catch (err) {
+            logging.error(err);
+            return false;
+        }
+
+        return true;
+    }
+
+    async getSuppressionData(email) {
+        try {
+            const model = await this.Suppression.findOne({
+                email_address: email
+            });
+
+            if (!model) {
+                return new EmailSuppressionData(false);
+            }
+
+            return new EmailSuppressionData(true, {
+                timestamp: model.get('created_at'),
+                reason: model.get('reason') === 'spam' ? 'spam' : 'fail'
+            });
+        } catch (err) {
+            logging.error(err);
+            return new EmailSuppressionData(false);
+        }
+    }
+
+    async getBulkSuppressionData(emails) {
+        if (emails.length === 0) {
+            return [];
+        }
+
+        try {
+            const collection = await this.Suppression.findAll({
+                filter: `email_address:[${emails.join(',')}]`
+            });
+
+            return emails.map((email) => {
+                const model = collection.models.find(m => m.get('email_address') === email);
+
+                if (!model) {
+                    return new EmailSuppressionData(false);
+                }
+
+                return new EmailSuppressionData(true, {
+                    timestamp: model.get('created_at'),
+                    reason: model.get('reason') === 'spam' ? 'spam' : 'fail'
+                });
+            });
+        } catch (err) {
+            logging.error(err);
+            return emails.map(() => new EmailSuppressionData(false));
+        }
+    }
+
+    async init() {
+        const handleEvent = async (event) => {
+            try {
+                await this.Suppression.add({
+                    email_address: event.email,
+                    email_id: event.emailId,
+                    reason: 'bounce',
+                    created_at: event.timestamp
+                });
+            } catch (err) {
+                if (err.code !== 'ER_DUP_ENTRY') {
+                    logging.error(err);
+                }
+            }
+        };
+        DomainEvents.subscribe(EmailBouncedEvent, handleEvent);
+        DomainEvents.subscribe(SpamComplaintEvent, handleEvent);
+    }
+}
+
+module.exports = MailgunEmailSuppressionList;