Views grouping field is double-escaped

Problem/Motivation

Views grouping field is not rendered properly if the field has any HTML special characters. For example, an apostrophe is rendered as '

Steps to reproduce

1. Create a node with an apostrophe in its title
2. Create a view displaying nodes
3. Configure your view to group by node title
4. In a view either add a label to your node title field or tip the "Remove tags from rendered output" checkbox in the grouping settings
5. It will replace an apostrophe with a corresponding HTML entity ' when the grouping field is rendered

Proposed resolution

Grouping field variable should be a Markup object, not a string. In that case the twig template will render HTML special characters properly.

Remaining tasks

test, review the patch, propose a better solution.

Release notes snippet

HTML special characters are rendered correctly in Views grouping.