If Windows or Linux, variable `userOSKey` will be 'Ctrl'. For Macs, `userOSKey`
will be '⌘' and these values will show up in place of the previous 'Ctrl/Cmd' keys.
Finished adding all keyboard shortcuts to the Settings page. Styled the tables
such that they are uniform with their columns being the same
width.
At the bottom of the 'Keyboard Shortcuts' settings page, a link to the
complete keyboard shortcuts documentation
(https://chat.zulip.org/help/keyboard-shortcuts) was also added.
This is the initial pass at adding keyboard shortcuts to the Settings page. In
this commit, the `ShortcutsSection` class has been created and is applied to a
newly-added 'Keyboard Shortcuts' nav item. The template for ShortcutsSection
is essentially multiple settings cards containing one table of keyboard short-
cuts organized by their underlying functionality.
The HTML `<kbd>` tag was defined in preference.css which styles the element to
look like a keyboard key, similar to StackOverflow or GitHub.
