fix: Setup Sentry only when Desktop is onboarded

[taratatach]

We need a valid Desktop config to setup our Sentry integration as we
use the Cozy URL to determine the domain, the instance and the
environment of the Cozy to tag our events.

Since we want to receive events as soon as possible, we setup Sentry
very early and, when Desktop is not onboarded already, we get an error
since we don't have the required data.
This is not a big deal but the error is printed in the console.

We'll now make sure we have a valid configuration before trying to
setup the integration and we'll also make sure to set it up after the
onboarding is done to receive events without the need for an app
restart.

Please make sure the following boxes are checked:

• PR is not too big
• it improves UX & DX in some way
• it includes unit tests matching the implementation changes
• it includes scenarios matching a new behaviour or has been manually tested
• it includes relevant documentation