Introduce environment variable to control dovecot full-text-search

[tremlin]

What type of PR?

Enhancement

What does this PR do?

In #1320 a full-text-search feature was enabled in Dovecot by default. Since this can have a big impact on performance, I think it's preferable to offer an option to disable the feature if it is not needed. This PR doesn't change the default behavior (FTS on).

Related issue(s)

• Add xapian full-text-search plugin to dovecot #1320

Prerequistes

Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

• In case of feature or enhancement: documentation updated accordinagly
• Unless it's docs or a minor change: add changelog entry file.

[mergify]

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[ofthesun9]

Thanks for your PR.
Could you please amend it to add a description in the advanced settings in the various .env templates:

• docs/compose/.env
• setup/flavors/compose/mailu.env

And/or in docs/configuration.rst

[tremlin]

Thanks for your suggestion. I tried to add some basic documentation.

[Nebukadneza]

This looks simple and great, with the docs added, i’d love this for 1.8-rc! Thanks!

[HorayNarea]

Looks good, approved in meeting with @Nebukadneza :D

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