Improve UX of adding a third party app as trusted

Current UX with NSS 5 for using a third-party webapp:

• visit e.g. https://pheyvaer.github.io/solid-chess/
• click 'Log In'
• a popup gives you four options, pick 'custom'
• enter your WebId, or at least the protocol+hostname of it
• log in to your pod (if you're not already logged in)
• the popup wil close and the app will say you're logged in
• open a separate tab, browse to your own WebId (make sure to include the '#me')
• hover over the 'card' title
• select the 'A' icon
• enter the origin of the app, https://pheyvaer.github.io
• tick the read, write, and append boxes
• click add
• switch back to the app pane and start using the app

Proposal: add a checkbox that makes it easier to add an app as trusted, as part of the login flow. With this, it would become:

• visit https://pheyvaer.github.io/solid-chess/
• click 'Log In'
• a popup gives you four options, pick 'custom'
• enter your WebId, or at least the protocol+hostname of it
• log in to your pod (if you're not already logged in)
• if the app is not in your list of trusted apps yet, you will see three buttons in the popup dialog: 'identity only (don't add as trusted app)', 'identity+read only', and 'identity+read+write'
• click one of those buttons
• the popup wil close and the app will say you're logged in
• no need to go into the data browser's trusted apps pane separately