Fix #4999: Unexpected error: ActiveRecord::RecordNotUnique sometimes appears when uploading posts

Fix two issues that could lead to duplicate errors when creating posts:

* Fix the submit button on the upload form to disable itself on submit, to prevent
  accidental double submit errors.

* Fix a race condition when checking for MD5 duplicates. MD5 uniqueness is checked on both
  the Rails level, with a uniqueness validation, and on the database level, with a unique
  index on the md5 column. Creating a post could fail with an ActiveRecord::RecordNotUnique
  error if the uniqueness validation in Rails passed, but the uniqueness constraint in the
  database failed. In this case, we catch the RecordNotUnique error and convert it to a
  Rails validation error so we can treat it like a normal validation failure.

app/views/uploads/show.html.erb

@@ -92,7 +92,7 @@
           <%= render "related_tags/buttons" %>
         </div>
 
-        <%= f.submit "Upload", id: "submit-button", data: { disable_with: false } %>
+        <%= f.submit "Post" %>
 
         <% if CurrentUser.can_upload_free? %>
           <%= f.input :is_pending, as: :boolean, label: "Upload for approval", wrapper_html: { class: "inline-block" }, input_html: { checked: @post.is_pending? } %>