compose_banners: Explicitly target banner content, child p.

This ensures uniform display and flexing of different bits of
banner content, including:

* Banners like Sent! Scroll down to view...) that have only a
  classless `<p>` marking their content
* Banners like the unscheduled message allert, which puts a
  .banner_content class right on the paragraph
* Banners like the Your message was sent to a stream you have
  muted", which tuck a `<p>` into a div with the .banner_content
  class

web/styles/compose.css

@@ -301,7 +301,8 @@
         flex-wrap: wrap;
     }
 
-    & p {
+    & .banner_content,
+    & > p {
         margin: 0; /* override bootstrap */
         /* 5px right padding + 10px left-margin of the neighbouring button will match the left padding */
         padding: 8px 5px 8px 15px;