switch to https:// redirect url for SSO login to improve "do you trust element://" UX

After you complete SSO, the server will ask if you trust the client, before it hands access to your client to an unknown url:


If the redirect URI is `https`, Synapse will just display the hostname, rather than the full url.

It is therefore proposed that, at least for macOS and Windows, element-desktop register an intercept for a url under the `https://element.io` heirarchy and then use that as the redirect URI. That will make synapse show "Continuing will grant `element.io` access to your account" instead of an opaque (to the average user) `element://` URI.