Add pagination to /admin/users/list API

Prior to this, only the first 100 active/new/etc. users were available
via the `/admin/users/list` API.  This change adds support for a
`page=#` querystring parameter so that *all* of the users can be
retrieved.  Requests for pages past the last user result in an
empty-list response; requests for negative pages (or zero) just return
the first page.

Added tests to cover pagination.

spec/components/admin_user_index_query_spec.rb

@@ -26,7 +26,7 @@ describe AdminUserIndexQuery do
       query = ::AdminUserIndexQuery.new({ order: "trust_level" })
       expect(query.find_users_query.to_sql).to match("trust_level DESC")
     end
-    
+
     it "allows custom ordering asc" do
       query = ::AdminUserIndexQuery.new({ order: "trust_level", ascending: true })
       expect(query.find_users_query.to_sql).to match("trust_level ASC" )
@@ -43,6 +43,28 @@ describe AdminUserIndexQuery do
     end
   end
 
+  describe "pagination" do
+    it "defaults to the first page" do
+      query = ::AdminUserIndexQuery.new({})
+      expect(query.find_users.to_sql).to match("OFFSET 0")
+    end
+
+    it "offsets by 100 by default for page 2" do
+      query = ::AdminUserIndexQuery.new({ page: "2"})
+      expect(query.find_users.to_sql).to match("OFFSET 100")
+    end
+
+    it "offsets by limit for page 2" do
+      query = ::AdminUserIndexQuery.new({ page: "2"})
+      expect(query.find_users(10).to_sql).to match("OFFSET 10")
+    end
+
+    it "ignores negative pages" do
+      query = ::AdminUserIndexQuery.new({ page: "-2" })
+      expect(query.find_users.to_sql).to match("OFFSET 0")
+    end
+  end
+
   describe "no users with trust level" do
 
     TrustLevel.levels.each do |key, value|