swap smex major mode commands and M-x

not using smex-major-mode-commands much, so move that to M-X and return
execute-extended-command back to original keybinding
tweak citation line format
function to locate and follow gnus link

For articles with many links, it can get annoying to press
<TAB> (gnus-summary-widget-forward) to get to the external link to the
current article.