Switch to voila as a Jupyter Server ExtensionApp

[jtpio]

Problem

With the ExtensionApp now part of the latest Jupyter Server stable releases since 1.0, we should switch voila to using it.

Proposed Solution

There has already been a couple of PRs for this:

• [WIP] Voila as an ExtensionApp #270
• refactor: use jupyter_server ExtensionApp #492
• Voila as an ExtensionApp (was #270) #563
• Use ExtensionApp #592

We could pick the more recent PR, rebase it, fix conflicts and continue from there.

Or start from scratch with a new PR (since there are a lot of conflicts), adding everyone who contributed before to the commits as co-authors.

Additional Context

We can track that for the 0.3.0 since it will most likely introduce some breaking changes.

[jtpio]

cc @davidbrochart since you were the last one attempting such update, do you think it's better to pick up #592 or start over?

Also this is something that could be nice to have if we start having a fps plugin at some point: #977

[davidbrochart]

Given the amount of conflicts, I think it is better to start over.