This removes the old base button style which was a blue button and
kills the unnecessary .white class which was essentially just acting as
the new button base.
This then removes all references throughout the settings/subscriptions
pages to those button styles.
This also fixes the strange button styles that changed the :hover and
:active opacity to 0.05 which led to unpredictable results on various
backgrounds.
This restructures the styling for the Zulip settings and
administration pages to minimize use of Bootstrap and use a consistent
styling library for similar elements.
While it is basically a wash in terms of the page's visuals, it will
make our life a lot easier for future work on improving the settings
pages section of the site.