Persist the sharing service JWT token across browser sessions

[agriyakhetarpal]

As discussed with @ivonnem3, we are in a position to, and should now, start looking into persisting the token safely in the browser cookies instead of regenerating one for each session, so that loading a notebook with a shared ID (see also #104) from a different tab, reloading the page, etc. does not make the notebook View Only.

This should be handled either after #104 or in conjunction with it.

[agriyakhetarpal]

I've reduced the priority of this issue to "low", as we have a way forward with #104 that we think we should work.