Fix missing message margin regression

I'm not sure when or where this came from, but it's probably worth
fixing before merging this branch into `master`. This resolves the issue
by moving the message header margins into the base theme and adding a
margin to the `<section>` tag.

Author: christianbundy

styles/base/message.mcss

@@ -10,6 +10,7 @@ Message {
     padding: 0 2px
   }
   header {
+    margin: 15px 20px 0
     font-size: 120%
     display: flex
     div.mini {
@@ -260,6 +261,7 @@ Message {
       }
     }
     section {
+      margin: 0 15px
       font-size: 90%
       font-style: italic
       color: #777

styles/dark/message.mcss

@@ -5,7 +5,6 @@ Message {
     color: #674e00
   }
   header {
-    margin: 20px 20px 0
     div.main {
       div.main {
         div.name {

styles/light/message.mcss

@@ -5,7 +5,6 @@ Message {
     color: white
   }
   header {
-    margin: 15px 20px 0
     div.main {
       div.main {
         div.name {