Disable "Reset Mirrors to Defaults" button when the list is already at the default

[jacobgkau]

When X-Repolib-Default-Mirror is set, we show a "Reset Mirrors to Defaults" button:

That button should probably be disabled when URIs is equal to X-Repolib-Default-Mirror, both to make it clear that it won't do anything in that case, and to provide more solid feedback after it's been used. (Disabling the button is probably better than hiding it, so users know they'll have the option of resetting to defaults before they start changing things.)

As of right now, the button prompts me for authentication even when it won't do anything, and I'm able to continue clicking it to no effect (when the list is already set to the default.)