[ticket/10896] Adds email validation to email settings in ACP

Adds a new validation type to the ACP validate_config_vars function
and implements it on the board_contact and board_email settings.

PHPBB3-10896
[ticket/10896] Move EMAIL_INVALID_EMAIL to common, replace EMAIL_INVALID

PHPBB3-10896
[feature/migrations] Make the container available to extension installers

This allows extensions to load and install migrations easily as per their
needs.

PHPBB3-11318
[feature/migrations] Automatically install/revert migrations for extensions

Migrations from ext/ext_name/migrations/ are automatically installed
when enabling the extension and automatically reverted when the
extension is purged.

PHPBB3-11318
[feature/migrations] Fix path to extension migrations

PHPBB3-11318
Merge branch 'develop' of https://github.com/phpbb/phpbb3 into feature/migrations-extensions

# Via Igor Wiedler (1) and Nathaniel Guse (1)
* 'develop' of https://github.com/phpbb/phpbb3:
  [ticket/11350] Do not pass $db by reference; typehint phpbb_db_driver
[feature/migrations] Inject Migrator instead of using the container to fetch

PHPBB3-11318
[feature/migrations] Call revert correctly when purging an extension

PHPBB3-11318
[feature/migrations] Add language strings for migrations errors

Unfulfillable returns string of the missing dependency name now if
the migration is unfulfillable (this is significantly more helpful).

PHPBB3-11351
[feature/migrations] Add explanatory language string for migration errors

This should be used if an exception is thrown to inform the user of
what occurred.

PHPBB3-11351
[feature/migrations] Catch and display errors from the migrator

PHPBB3-11318
[feature/migrations] getParameters function for migration exception

PHPBB3-11351
[feature/migrations] Fix unfulfillable function

Returned unfulfillable name in the wrong place previously

PHPBB3-11351
[feature/migrations] Fix failing tests

PHPBB3-11318
[feature/migrations] Fix failing tests (again)

PHPBB3-11318
[feature/migrations] getLocalisedMessage function for migration exception

PHPBB3-11351
[feature/migrations] Use getLocalisedMessage() function to get error message

PHPBB3-11318
[ticket/11359] close span

PHPBB3-11359
[ticket/11359] add id attribute to textarea

PHPBB3-11359
[ticket/11359] html escape sphinx config data

PHPBB3-11359