Fix Remove button on "Access Groups" page

The URL was missing, and the class should include btn-danger.
Merge pull request #1238 from jaysoffian/fix-remove-access-group-btn

Fix Remove button on "Access Groups" page
Note about how to start workers as its own topic
Remove unused reservoir sampling code
Merge pull request #1245 from jaysoffian/remove_pool_code

Remove unused reservoir sampling code
R Configuration DSN quote comment fix
Merge pull request #1248 from x0xMaximus/master

R Configuration DSN quote comment fix
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
Add new "remove_event" permission

This permission may be used to disallow event removal, without
disallowing the other event actions (Mute, Share, etc). Since
this is a new permission, we fall back to "admin_event" if
"remove_event" is not False.
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".
Merge pull request #1252 from jaysoffian/sentry_enable_invites

Make sending email invites configurable
Merge pull request #1251 from jaysoffian/remove_event_perm

Add new "remove_event" permission
Various improvements to logging/imports
lol
Allow superuser status to be set on Admin page
Merge pull request #1253 from jaysoffian/admin_superuser

Allow superuser status to be set on Admin page