Config value to allow to extend .user.ini on update

[makoehr]

When running nextcloud with a web hoster it might be necessary to extend .user.ini after each update (e.g. adding memory_limit). To automate this step, an additional config entry may be provided in config.php that specifies the lines to be added to .user.ini.

If the config option 'user_ini_additional_lines' exists, the provided value (string or array of strings) will be added to the file .user.ini. This config option has to be processed by the updater.
This is related to nextcloud/updater#471

Summary

Improved update for users with a web hoster

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are not required
• Screenshots before/after for front-end are not required
• Documentation (manuals or wiki) has been updated or is not required

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