remove raw cell placeholder on focus, closes #5238
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
fallback on copy, copyfile if copy2 fails

in notebook manager
remove some redundant font-family: monospace

`pre`, `code` tags already set the font-family in these contexts.
Setting it again in the surrounding context is redundant,
and causes inconsistency when embedding notebooks in HTML
(e.g. nbviewer).
rename css class names to be consistent with current style
fab css
elide long containers in pretty

limits sequence length to 1k items

avoids performance issues with pretty-printing large containers
simplify truncated enumeration in pretty

thanks @ivanov!
make max_seq_length an attr on PrettyPrinter

instead of module global
make _enumerate a method on PrettyPrinter

even less duplicate code
allow max_seq_length=0 to disable truncation altogether
don't instantiate Application just for default logger

It's unlikely to happen in IPython itself,
but can happen if config loaders are used elsewhere.
Remove \DefineShortVerb[commandchars=\\\{\}]{\|}
Fix import of socketserver on Python 3.

Closes gh-5382
Merge pull request #5379 from minrk/config-grr

don't instantiate Application just for default logger
render on focus out, always pad raw cells
raw cell placeholder text as one line