Change textareas to resize vertically only

This selector matches these elements directly:
  - "View Markdown" source on topics and comments
  - Filtered topic tags

And these elements indirectly via `{{ markdown_textarea() }}`:
  - New topics and comments
  - Edits of topics and comments
  - Group wiki editing
  - Direct messages

In all cases the width is already constrained by CSS.  No other layout
changes are necessary.

tildes/scss/modules/_form.scss

@@ -94,7 +94,7 @@ select.form-select:not([multiple]):not([size]) {
 textarea.form-input {
   height: 8rem;
   line-height: 1.5;
-  max-width: none;
+  resize: vertical;
   transition: none;
 }