Add restorer to code snippet extension

[marthacryan]

Adds behavior of reopening the code snippets extension on reloading
jupyter lab (if code snippets was already open). Fixes #432

Developer's Certificate of Origin 1.1

   By making a contribution to this project, I certify that:

   (a) The contribution was created in whole or in part by me and I
       have the right to submit it under the Apache License 2.0; or

   (b) The contribution is based upon previous work that, to the best
       of my knowledge, is covered under an appropriate open source
       license and I have the right under that license to submit that
       work with modifications, whether created in whole or in part
       by me, under the same open source license (unless I am
       permitted to submit under a different license), as indicated
       in the file; or

   (c) The contribution was provided directly to me by some other
       person who certified (a), (b) or (c) and I have not modified
       it.

   (d) I understand and agree that this project and the contribution
       are public and that a record of the contribution (including all
       personal information I submit with it, including my sign-off) is
       maintained indefinitely and may be redistributed consistent with
       this project or the open source license(s) involved.

[karlaspuldaro]

LGTM

[ajbozarth]

Ran locally and this LHTM. Though I did notice a lab bug while testing. The layout restorer doesn't restore the state of the right pane. I'll file a lab issue (and see if it's a quick fix)

[ajbozarth]

The bug I mentioned is a know issue in lab jupyterlab/jupyterlab#7225 shouldn't block this PR though