Fix spacing for callout titles

[froggleston]

This PR is a work in progress (as in it hasn't been tested anywhere near enough) to fix #562 with spacing in callout title headers. Currently, any HTML tags within the h3 title will lose any spacing due to the removal of whitespace when checking for translations.

The fix moves the trimming to inside the translation application function (apply_transformations) so leaves the tag spacing unchanged when replacing any translated elements. However, this hasn't been thoroughly checked.

[froggleston]

The fix is more complicated it seems as translation messes with what strings are sanitised and matched:

• Any string prior to translation is stripped of whitespace, to ensure exact matching.
• However, h3 titles with <code> blocks have trailing/leading whitespace also removed, leading to the issue described.

I've addressed this by moving the check and removal of whitespace to inside the apply_translation function, allowing us to retain knowledge of if the string did indeed have whitespace and to put it back if removed.

Similarly, I have excluded h3 titles for the translation check (and whitespace handling) which have inner tags, like <code>. Whilst this isn't optimal, I'm unsure of how to improve it without using some sophisticated matching for translation strings.

[ErinBecker]

I can confirm that this works for English-language lessons (tested with Foundations of Astronomical Data Science and Collaborative Lesson Development Training (screenshots below))

Agreed with @froggleston that this should also be tested on lessons that go through the translation process.

[tobyhodges]

I have tested this on a local version of a lesson in english and the version of git-novice localised to Japanese. It looks good in both cases (screengrab from the Japanese lesson build below) so I think this is ready to merge.