Fix auto-wrap for long, unbreakable text

These new CSS rules apply @syncguru's original solution [1] in a more
selective way. The main problem with the initial approach was its
application to the whole table.

As CSS tables are divided in two main parts (header <thead> and body
<tbody>), we can restrict the rule to <tbody>, which ensures that the
the header's behavior is not affected.

Fixes 0027114, PR https://github.com/mantisbt/mantisbt/pull/1761

[1]: see PR https://github.com/mantisbt/mantisbt/pull/1690,
reverted in 60d28cce0569d2acd1daccce930e5215a1abda72

Co-authored-by: Malo MM. MAISONNEUVE mmaisonneuve@cantoriel.local