Use LabIcon in code-snippet package

[ajbozarth]

Update the icon usage in code snippets to use LabIcons

This includes:

• Moving the code-snippet logo into application
• tweaking the svg viewBox to remove excess space around the icon
  which make it properly fill the width off the icon in the css
• replace all icons set with css with LabIcon instances
• some minor icon css size tweaking

Fixes #443
Fixes #430

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[karlaspuldaro]

We should look into failing tests, doesn't look like they're caused by your changes.
Otherwise, LGTM.

[ajbozarth]

The tests actually consistently fail locally as well, I'm taking a look to figure out why

[ajbozarth]

I've tracked down the cause: It's the same issue as has been raised in the git extension jupyterlab/jupyterlab-git#604
I'm testing a work around but given this is now hitting us twice I'm going to open a PR on lab with my idea I suggested in the git pr

[ajbozarth]

I've pushed a workaround fix for the tests and will be opening a PR on lab to fix it there since it's also happening to jupyterlab-git