content: Use directional layout for message media

Previously, these previews (for images and video) were hardcoded
to align to the left, with padding on the right. This resulted
in an incorrect layout in RTL locales.

We now use directional positioning so that the alignment and
padding flip correctly based on the text direction.

Suggested by @gnprice.

Author: yash-agarwa-l

lib/widgets/content.dart

@@ -741,11 +741,11 @@ class MessageMediaContainer extends StatelessWidget {
     return GestureDetector(
       onTap: onTap,
       child: UnconstrainedBox(
-        alignment: Alignment.centerLeft,
+        alignment: AlignmentDirectional.centerStart,
         child: Padding(
           // TODO clean up this padding by imitating web less precisely;
           //   in particular, avoid adding loose whitespace at end of message.
-          padding: const EdgeInsets.only(right: 5, bottom: 5),
+          padding: const EdgeInsetsDirectional.only(end: 5, bottom: 5),
           child: ColoredBox(
             color: ContentTheme.of(context).colorMessageMediaContainerBackground,
             child: Padding(