jquery.suggestions, mediawiki.searchSuggest: Fix form submission

Don't override form submission behavior (by capturing the Enter key on
search box, preventing default behavior, and then manually submitting
the form); just let it happen if it's wanted.

Then swap out a few unjustified stopImmediatePropagation() calls to
stopPropagation() so that the event can be actually fired. Add some
comments while we're at it.

This allows the user to use Ctrl+Enter (or Shift+Enter) to submit the
form into a new tab (or new window) on browsers that support this
(currently Opera and Chrome).

Bug: 34756
Bug: 35974
Change-Id: I49ef7cc89400032505bc444f21d522d5b5d47586