Shows the "load more tweets" button once a set of tweets is loaded.

fixes #29.
moved the load more button to the toolbar.

related #29
Add missing i18n to new 'load more tweets' label.
Only show Twitter 'load more' button for screens where the user has made searches.

Prevents the 'load more' button being clickable while an AJAX request
is in progress, and also prevents the button showing up when you go to
a new tab. See #29.

Most of this is a temporary change until we integrate Infinite Search
for the Twitter service, see #39.
Merge pull request #43 from Automattic/twitter-loadmore

Fix Twitter service 'load more' button.  Props @jlgarhdez, @paulgibbs