Add invite button to admin Users page header

Ref T694

less/admin.less

@@ -48,3 +48,10 @@ nav#admin {
 		}
 	}
 }
+
+.admin-actions {
+	.btn {
+		font-family: @sansFont;
+		font-size: 0.86em;
+	}
+}

less/core.less

@@ -1421,7 +1421,7 @@ div.row {
 }
 
 @media all and (max-width: 600px) {
-	div.row {
+	div.row:not(.admin-actions) {
 		flex-direction: column;
 	}
 	.half {

templates/user/admin/users.tmpl

@@ -4,7 +4,10 @@
 <div class="snug content-container">
 	{{template "admin-header" .}}
 
-	<h2 id="posts-header" style="display: flex; justify-content: space-between;">Users <span style="font-style: italic; font-size: 0.75em;">{{.TotalUsers}} total</span></h2>
+	<div class="row admin-actions" style="justify-content: space-between;">
+		<span style="font-style: italic; font-size: 1.2em">{{.TotalUsers}} {{pluralize "user" "users" .TotalUsers}}</span>
+		<a class="btn cta" href="/me/invites">+ Invite people</a>
+	</div>
 
 	<table class="classy export" style="width:100%">
 		<tr>