Prevent resubmit same suggestions

Adding "filter out resubmit / declined string" filter in IMPORT page and translate page.

Why need it?
translator export their .po from own site and upload to LDO, they keep uploading it every time when they updated their .po files. However, their .po files may contains bad suggestions or their own style. that would cause same declined string appear in LDO UI again.

and it should be used by DEFAULT, users never read guideline before they do something.

it's an enhanced version of #730674: Warn when resubmitting same strings