Auto update application

[pverscha]

Automatically update the desktop application when a new version is available. The new version is downloaded and installed automatically in the background without the need for user intervenience.

The user is informed about the download process with a snackbar at the bottom of the application:

In case of a successful installation the user is greeted with a different message:

Errors that may occur during application updates are intercepted and presented to the user:

[pverscha]

This PR also includes a GitHub action that automatically builds a new version of our application after each push to master.

[pverscha]

This PR should not be merged before auto-updating works on macOS. For this to work, we are required to notarise our application. This can only be done by Apple Developers (which requires us to pay a 100$ registration fee each year). Notarising our app also removes the GateKeeper warning that appears when users try to install Unipept Desktop.

For Windows, we are also required to sign the app using an EV-certificate in order to get rid of the "Windows protected your PC" message that obstructs installation.

Both notarisation and signing are not mandatory, but will discourage users to install the app.