Add a MarkdownInput widget, and make use of it

Signed-off-by: Skia <skia@libskia.so>

core/static/core/style.scss

@@ -515,6 +515,12 @@ textarea {
       margin: 1px;
       font-size: smaller;
     }
+    a {
+      color: $black-color;
+    }
+    a:hover {
+      text-decoration: underline;
+    }
   }
 
   .tools {
@@ -551,6 +557,12 @@ textarea {
       margin: 1px;
       font-size: smaller;
     }
+    a {
+      color: $black-color;
+    }
+    a:hover {
+      text-decoration: underline;
+    }
   }
 
   .category {