Fix shell.openExternal calls now that it's an async method

[kuychaco]

With the upgrade to Electron 5 shell.openExternal is now an async method (see electron/electron#16176).

So we want to update the following accordingly...

desktop/app/src/main-process/main.ts

Lines 483 to 484 in e23328a

	const result = shell.openExternal(path)
	event.sender.send('open-external-result', { result })

As it is now, we prematurely send the 'open-external-result' event. And we won't be alerted if this operation fails, as we're not handling the promise rejection. From what I can tell we're not actually doing anything with result, so nothing is breaking due to the fact that its value is now a promise rather than a boolean, as was previously returned from shell.openExternal.

To fix this we can either use shell.openExternalSync or await the promise that is now returned from shell.openExternal.

Note that strangely shell.openExternalSync returns a boolean whereas shell.openExternal returns Promise<void>. I'm generally in favor of using the async method, unless we rely on the boolean return value for subsequent logic in our code.

There are several other places that we use shell.openExternal in the code, so we'll want to do a thorough audit and make sure to use the appropriate method in each case.

/cc @vilmibm