Theme: Break words in paragraphs and headings only

[EricDunsworth]

#2450 added a CSS selector that set the word-break property to break-word anywhere within the main element.

But it had unexpected side effects in certain kinds of two-dimensional content, such as tables and input groups.

Specifically:

• It could cause wide tables (even ones situated within table-responsive containers) to divide their columns improperly - resulting in individual numbers in number groupings "splitting" across lines, columns that are too narrow to fit short words on the same line, non-breaking spaces getting ignored, etc...
• Buttons situated in input groups became "squished" - every letter within affected buttons appeared on its own line

This resolves it by reworking the selector to solely target paragraphs and headings. Paragraphs in table cells are exempted.

Fixes #2466.

CC @Garneauma @duboisp

[EricDunsworth]

Btw I wanted to modify the original selector rule from main * to something along the lines of main :not(table, table *) to avoid needing an "undo" override... but it didn't work in practice since word-break is an inherited property.

For example, if the main element contains a div and a table is situated within... the table and its children will still inherit the div's word-break: break-word; property - even though the CSS selector doesn't directly apply to them.

There's no way around the need for an undo override. But if there's an appetite to fix the issue + keep word-break functioning as an inherited property on the table's children... it'd be possible to achieve via main :not(table *) {word-break: break-word;} and main table {word-break: initial;}.

@Garneauma @duboisp Would you prefer that I update this PR to use the alternate way? IMO it'd be better overall, at the cost of making the first selector slightly-more complex.

PS:
#2450 also negatively impacted the timers in the WET multimedia player's controls in some edge cases...

I was originally going to cover .wb-mm-txtonly * in the undo selector rule to fix it, but it turns out that GCWeb already has a media player override that does the trick.

[EricDunsworth]

This PR will probably need to be extended to cover input groups and maybe even buttons in general (e.g. .btn, button, input[type="submit"]).

Selectors that apply the display: table; and display: table-cell; properties might also need to be double-checked just in case.

[EricDunsworth]

Revised the PR as follows:

• Merged @Garneauma's suggestion
• Pushed another commit to exempt paragraphs situated in table cells (paragraphs in table captions are still covered since they function similarly to "regular" paragraphs)
• Updated the PR's subject + OP to reflect what's changed and mention input groups