Poll history - make poll history independent from dialogs (#10349)

* move pollhistory from dialogs to polls directory

* rename PollHistoryDialog -> PollHistory

* rename references to PollHistoryDialog

* move title to PollHistory

* add option to collapse empty dialog header

res/css/views/polls/pollHistory/_PollHistory.pcss

@@ -14,10 +14,14 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-.mx_PollHistoryDialog_content {
+.mx_PollHistory_content {
     height: 600px;
     width: 100%;
 
     display: flex;
     flex-direction: column;
 }
+
+.mx_PollHistory_header.mx_Heading_h2 {
+    margin-bottom: $spacing-16;
+}