semantic names for indicator icons

For all of the discussion that we had about what kind of icons should
and should not be used to indicate what mode the notebook is in, we
never went through to make it possible to override it.

With this change, it is now possible to override what icons are
displayed for Command and Edit Modes.

For example, @minrk liked the fighter-jet icon for Command Mode, so he
can put this in his custom.css

.ipython-command-mode:before {
     content: "\f0fb";
}
semantic names for kernel status indicator
ensure ipython-command-mode icon displayed on load
better fix for starting with Command Mode icon
rename css class names to be consistent with current style
fab css
Merge pull request #5332 from ivanov/semantic-icons

semantic names for indicator icons