This is everywhere that I could just replace `<button className='btn
btn-primary'>` with `<Button emphasis='primary'>` (and some other
emphasis versions) without any additional changes. There's still more
places where this could be used which require more in-depth changes that
will be in a following commit.

This is a minor functional change because these elements are now
accessible.

This is removing some usage of a save-button CSS
that doesn't seem to affect these components.

There's a small functional change here because the copy button in the
header of the FullLogEventModal is now styled when it wasn't before.

… btn-danger

This removes some CSS classes from some different elements, but as
elsewhere, those CSS classes don't actually do anything. I think some
might have had a purpose once, but there seems to be quite a few that
were copied around during previous, possibly AI-assisted refactors.

Everything that used that prop either passed the tertiary class that
was the default or passed a class that didn't exist.

…aternary

These still need inverted button support, but I took the opportunity to
clean up some convoluted code to determine what styling they otherwise
receive.

All of these are anchors with click handlers and the btn class, so
they'd appear as buttons anyway.

This includes some minor functional changes due to moving these
components to standardized CSS classes. The styles mostly matched a
secondary button except for slightly different padding and colours.

I also removed a workaround for an old issue with OverlayTrigger and
disabled buttons in favour of just using the disabled attribute. For
more information on the previous code, see
mattermost/mattermost-webapp#10387. Based on
some brief testing, that's no longer needed.

It duplicates btn-danger with a slightly different colour in some cases

The places we passed btn-primary previously were redundant

…mButtonVariant