Make remaining dialogs in wiki accessible.

All the dialogs in the wiki are now accessible to screen readers.

Also, the "Switch to selected version" button did the odd thing of
reloading the page with the new version selected, but inside a dialog.
That is now fixed.

The merge dialog has a warning that looks kind of funky still, but you
can read it, unlike the previous code.

LMS-1337

lms/static/sass/course/wiki/_wiki.scss

@@ -536,8 +536,12 @@ section.wiki {
     }
 
     .modal-header {
+      border: 1px solid $danger-red;
+      padding: ($baseline/2) ($baseline/2) 0 ($baseline/2);
+      margin-bottom: ($baseline/2);
+
       h1, p {
-        color: #fff;
+        color: $base-font-color;
       }
 
       h1 {

lms/static/sass/shared/_modal.scss

@@ -59,7 +59,7 @@
     overflow: hidden;
     padding-left: 10px;
     padding-right: 10px;
-    padding-bottom: 30px;
+    padding-bottom: 10px;
     position: relative;
     z-index: 2;
 
@@ -136,7 +136,7 @@
 
     form {
       margin-bottom: 12px;
-      padding: 0px 40px;
+      padding: 0px 40px 20px;
       position: relative;
       z-index: 2;