refactor: add local timezone aware date format

[lorenzo-cavazzi]

The commit datetime showed to the user while launching a new notebook is now local timezone aware.

fix #571

[ciyer]

This is great! Could you add a test for toIsoTimezoneString (just 1 is fine)?

[lorenzo-cavazzi]

It turns out that it's not that easy to create a proper unit test because JavaScript doesn't allow to create a new date for a specific timezone https://stackoverflow.com/questions/15141762/how-to-initialize-a-javascript-date-to-a-particular-time-zone

I added a pretty long piece of code to test on the current user timezone. Basically, it works on string manipulation. This is the best way that came to my mind to avoid writing a test that is simply a re-write of the function to be tested. The other way would be adding an external library for time management, which doesn't look like a good way to test it either.

[lorenzo-cavazzi]

just rebased and squashed since it's a single topic