Make "Create a new user" button on Admin users page visible

The list.html template extends admin.html which overrides
the 'page_header_block' (from layout.html) with:

    {% block page_header_block %}
        <section id="page-header" class="toolbar" style="height:5px;">
        </section>
    {% endblock %}

Thus there is no 'page_header' into which list.html can insert
the button. Move the button into the lower button toolbar so
that it appears.
Merge pull request #1250 from jaysoffian/unhide-new-user-admin-button

Make "Create a new user" button on Admin users page visible
Make sending email invites configurable

Introduce "SENTRY_ENABLE_INVITES" setting to enable/disable the
ability to send email invites.

And while we're touching the new.html template, change the submit
button text for sending an invite from "Save Changes" to
"Send Invite".