Fix: show loading indicator after submitting an external connection URL

[dinhtungdu]

Description of the Change

Fixes #595.

Show a spinner and a message indicates that we're checking the connection.

Benefits

Make the UI more interactive.

Possible Drawbacks

Verification Process

Create a new connection using Auth Wizard, see the spinner and message after submitting the external site URL.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

[helen]

Have one string tweak and one other thing I noticed - the checking message shows even with an empty URL and then gets stuck there. I think it probably shouldn't show until it's past the URL checking a few lines later.

[dinhtungdu]

@helen Thanks for the review! Now the status only shows after URL and title are validated.

[helen]

Works great now!