Use Thymeleaf as a template engine for e-mails

[php-coder]

After updating Thymeleaf to v3 (#406) we can use new textual template mode for e-mails.

See:

• http://www.thymeleaf.org/doc/articles/springmail.html
• http://stackoverflow.com/questions/25610281/spring-boot-sending-emails-using-thymeleaf-as-template-configuration-does-not
• http://www.baeldung.com/spring-thymeleaf-3
• http://www.thymeleaf.org/doc/articles/thymeleaf3migration.html ("Textual template modes" chapter)
• https://github.com/thymeleaf/thymeleaf/issues/395
• http://blog.codeleak.pl/2017/03/getting-started-with-thymeleaf-3-text.html
• https://stackoverflow.com/questions/41599954/how-to-process-txt-e-mail-template-with-thymeleaf
• https://springhow.com/spring-boot-email-thymeleaf/

We should use unescaped ([(...)]) form in the templates.

[php-coder]

@cssru Are you interesting in taking this?

[cssru]

Yep, but I have one more task undone, as I recall.

[php-coder]

I'll continue on adding UI for sellers/buyers and another issue wasn't important. So, now, you have only one -- this one :)

[php-coder]

@cssru Are you working on it or I can re-assign to myself?

[php-coder]

Might be useful: https://github.com/jpomykala/spring-higher-order-components/issues/7