Show warning for username mismatch in credential popup and remote URL

[exactly-one-kas]

When a repository's remote URL contains an explicit username and the user enters a different username in the credentials popup, the password doesn't get saved to the credential store with the URL's username (as it should IMO)

Showing a message telling the user to consider changing the URL's username or removing it should improve that experience

For context, see #227

[exactly-one-kas]

Alternatively, pre-fill and grey out the username field when the URL contains a username