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.
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