rework how app mailboxes are allocated

Our current setup had a mailbox allocated for an app during app
install (into the mailboxes table). This has many issues:

* When set to a custom mailbox location, there was no way to access
  this mailbox even via IMAP. Even when using app credentials, we
  cannot use IMAP since the ldap logic was testing on the addon type
  (most of our apps only use sendmail addon and thus cannot recvmail).

* The mailboxes table was being used to add hidden 'app' type entries.
  This made it very hard for the user to understand why a mailbox conflicts.
  For example, if you set an app to use custom mailbox 'blog', this is
  hidden from all views.

The solution is to let an app send email as whatever mailbox name is
allocated to it (which we now track in the apps table. the default is in the
db already so that REST response contains it). When not using
Cloudron email, it will just send mail as that mailbox and the auth
checks the "app password" in the addons table. Any replies to that
mailbox will end up in the domain's mail server (not our problem).

When using cloudron email, the app can send mail like above. Any responses
will not end anywhere and bounce since there is no 'mailbox'. This is the
expected behavior. If user wants to access this mailbox name, he can
create a concrete mailbox and set himself as owner OR set this as
an alias.

For apps using the recvmail addon, the workflow is to actually create
a mailbox at some point. Currently, we have no UI for this 'flow'.
It's fine because we have only meemo using it.

Intuitive much!

migrations/schema.sql

@@ -80,6 +80,7 @@ CREATE TABLE IF NOT EXISTS apps(
     debugModeJson TEXT, // options for development mode
     robotsTxt TEXT,
     enableBackup BOOLEAN DEFAULT 1, // misnomer: controls automatic daily backups
+    mailboxName VARCHAR(128), // mailbox of this app
 
     // the following fields do not belong here, they can be removed when we use a queue for apptask
     restoreConfigJson VARCHAR(256), // used to pass backupId to restore from to apptask
@@ -173,12 +174,14 @@ CREATE TABLE IF NOT EXISTS mail(
 /* Future fields:
    * accessRestriction - to determine who can access it. So this has foreign keys
    * quota - per mailbox quota
+
+   NOTE: this table exists only real mailboxes. And has unique constraint to handle
+   conflict with aliases and mailbox names
 */
 CREATE TABLE IF NOT EXISTS mailboxes(
     name VARCHAR(128) NOT NULL,
     type VARCHAR(16) NOT NULL, /* 'mailbox', 'alias', 'list' */
-    ownerId VARCHAR(128) NOT NULL, /* app id or user id or group id */
-    ownerType VARCHAR(16) NOT NULL, /* 'app' or 'user' or 'group' */
+    ownerId VARCHAR(128) NOT NULL, /* user id */
     aliasTarget VARCHAR(128), /* the target name type is an alias */
     membersJson TEXT, /* members of a group */
     creationTime TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,