Center align "Save changes" button label in Options

[breadtk]

Description

Within the Options page, the #save button is displayed as "No changes"
(10 chars) on load. When onUpdated() is called in pages/options.js,
it turns into "Save changes" (12 chars). This commit ensures that the
update label is appropriately centered, even if that means less padding
on left/right, as opposed to having more padding on left/right in the
fixed size button.

[UncleSnail]

I think the preferred way to do this might be to remove the width property from the #save styles. That would mean that the button size can change though, so it depends on preference. However, this problem is likely to happen again in the future since the Add Rule button, and potentially other buttons, also share the #save style. It might be better just let them set their own width to avoid accidentally letting the text overflow. We could also use a min-width instead, and then future accidentally longer text wouldn't overflow.