Fix EZP-23712: The content edit form moves if you put the mouse over the title

JIRA: https://jira.ez.no/browse/EZP-23712

# Description

This patch fixes this annoying bug where the technical infos display on the content edit page changes the height of the header and makes the form to move. This was also the occasion to handle that with CSS rather than with JS code which makes the unit easier and quicker.

# Tests

manual tests + unit tests
I don't like the class name. What do you think about `tech-details-visible`?