Built-in scheduler to delete old emails and attachments

[alexbudarov]

Motivation:
https://www.cuba-platform.ru/discuss/t/otklyuchenie-sledov-zhiznedeyatelnosti-u-cuba-emailer/5396

Customer is complaining about growing size of the database.
95% of the database is consumed by emails and their attachments.

I think we need to have built-in cleaning scheduler out-of-the-box.
Scheduler should hard-delete messages, attachments from DB and associated files from file storage.

Settings:

• max age of messages
• some settings to distinguish unimportant messages to be deleted from important business letters that should be stored much longer or forever.

Addressing the problem of different kinds of messages
E.g. a system can send:

• booking confirmations and notifications - that aren't too important and can be deleted after some time
• salary documents, invoices - that are important and must be kept for many years or forever

In the simplest implementation we can define two kinds of messages: important or non-important.
Cleaning scheduler has two settings: max age of important messags (or forever), and max age of non-important messages.

Important / nonimportant, or "category" - this attribute can be set via a single Boolean / String attribute, or via set of tags.
Such attribute would be also convenient for categorizing / grouping messages in the browser of sent emails.

[Desire456]

Implementation

EmailCleaner hard-delete old email messages, their attachments from DB, and associated files from file storage by settings, which contains in the application.properties file:

• jmix.email.useDefaultEmailCleaningQuartzConfiguration - true if default Email Cleaning quartz scheduling configuration is used. By default that property is false and EmailCleaner is not working.
• jmix.email.emailCleaningCron - CRON expression that is used by default Email Cleaning quartz scheduling configuration.
• jmix.email.maxAgeOfImportantMessages - the maximum age (in days) of important messages after which they must be deleted. Zero value (0) means that messages won't be removed.
• jmix.email.maxAgeOfNonImportantMessages - the maximum age (in days) of messages after which they must be deleted. Zero value (0) means that messages won't be removed.
• jmix.email.cleanFileStorage - true if deleted from file storage is performed while the cleaning scheduler is working.