Use flex-start instead of start for postcss

This is just something the loaders complain about - apparently `start` is old and we should feel bad.

res/css/views/rooms/_MemberDeviceInfo.scss

@@ -17,7 +17,7 @@ limitations under the License.
 .mx_MemberDeviceInfo {
     display: flex;
     padding-bottom: 10px;
-    align-items: start;
+    align-items: flex-start;
 }
 
 .mx_MemberDeviceInfo_icon {

res/css/views/rooms/_MessageComposer.scss

@@ -101,7 +101,7 @@ limitations under the License.
     display: flex;
     flex-direction: column;
     min-height: 60px;
-    justify-content: start;
+    justify-content: flex-start;
     align-items: flex-start;
     font-size: 14px;
     margin-right: 6px;