CSS fixes.

Move styles related to embedded chats to `css/converse-muc-embedded.css`

css/converse-muc-embedded.css

@@ -16,6 +16,10 @@
     -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
     box-sizing: border-box; }
+  #converse-embedded-chat form.pure-form.converse-centered-form {
+    position: absolute;
+    top: 30%;
+    transform: translateY(-50%); }
   #converse-embedded-chat .chatroom {
     width: auto; }
   #converse-embedded-chat .flyout {
@@ -52,7 +56,6 @@
       font-size: 180%;
       float: left;
       height: 100%;
-      width: 100%;
       position: relative; }
       #converse-embedded-chat .chatroom .box-flyout .chatroom-body .chatroom-form-container input {
         font-size: 60%; }

css/converse.css

@@ -1368,9 +1368,6 @@
         color: #818479; }
   #converse-embedded-chat form.pure-form.converse-centered-form,
   #conversejs form.pure-form.converse-centered-form {
-    position: absolute;
-    top: 30%;
-    transform: translateY(-50%);
     text-align: center;
     width: 100%; }
   #converse-embedded-chat .chat-textarea-chatbox-selected,
@@ -2363,6 +2360,7 @@
         color: #818479;
         font-size: 14px;
         height: 289px;
+        width: 100%;
         height: -webkit-calc(100% - 55px);
         height: calc(100% - 55px);
         overflow-y: auto;

sass/_chatrooms.scss

@@ -137,6 +137,7 @@
                     color: $text-color;
                     font-size: $font-size;
                     height: 289px;
+                    width: 100%;
                     @include calc(height, '100% - #{$chat-head-height}');
                     overflow-y: auto;
                     position: absolute;

sass/_core.scss

@@ -196,9 +196,6 @@
             }
         }
         &.pure-form.converse-centered-form {
-            position: absolute;
-            top: 30%;
-            transform: translateY(-50%);
             text-align: center;
             width: 100%;
         }

sass/_muc_embedded.scss

@@ -12,6 +12,14 @@
     position: relative;
     width: 100%;
 
+    form {
+        &.pure-form.converse-centered-form {
+            position: absolute;
+            top: 30%;
+            transform: translateY(-50%);
+        }
+    }
+
     .chatroom {
         width: auto;
     }
@@ -63,7 +71,6 @@
                     font-size: 180%;
                     float: left;
                     height: 100%;
-                    width: 100%;
                     position: relative;
                     input {
                         font-size: 60%;