Config editor does not properly escape html tags in the edit form

Problem/Motivation

When editing a config file with HTML tags in it, the form is broken. We cannot save it either.

The <pre> tag does not escape HTML inside of it so those HTML occurrences must be escaped.

It happens on the devel.config_edit route and the devel config definition pages.

To reproduce this bug, edit a text filter such as filter.format.basic_html which contains restricted html tags.

Proposed resolution

1. Escape tags in the pre