Replace #concept-info-before div element with more generic #content-top, adding #content-bottom div element

[osma]

Reasons for creating this PR

In PR #1285, new empty DIV elements #concept-info-before and #concept-info-after were added as placeholders so that plugins can add widgets into these predefined slots. But these are specific to the concept page and not available on other page types such as the vocabulary home page. This PR replaces those DIVs with more generic ones #content-top and #content-bottom which can be used on all (?) page types.

Link to relevant issue(s), if any

• Follow-up fix to Call plugin callbacks in the order they are configured #1285

Description of the changes in this PR

• remove #concept-info-before and #concept-info-after DIV elements
• add #content-top and #content-bottom DIV elements
• some reindentation

Known problems or uncertainties in this PR

n/a

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if not, explain why below)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

Tests are not relevant since only Twig templates were changed.

[osma]

Tests on PHP 7.x are failing for some reason that is clearly unrelated to the changes in this PR:

PHPUnit\Framework\Exception: Fatal error: Uncaught TypeError: Argument 3 passed to PHP_Token::__construct() must be an instance of PHP_Token_Stream, instance of PHPUnit\PHP_Token_Stream given, called in phar:///usr/local/bin/phpunit/php-token-stream/Token/Stream.php on line 131 and defined in /app/vendor/phpunit/php-token-stream/src/Token.php:37

[sonarcloud]

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