Make errata more visible on the website

It's really hard to spot that a paper has an erratum on the website. For example, how long does it take you notice that [2021.emnlp-main.41](https://aclanthology.org/2021.emnlp-main.41/) has one?

A simple improvement would be to turn the erratum link into a button, similar to what attachments have. Maybe we also want it to appear as a big button on the right-hand side.

Start with the example paper page, 2021.emnlp-main.41, and inspect the website code that renders its erratum link alongside attachments. The work is done when the erratum is visibly presented as a button, with the possible right-hand-side placement resolved and the example page making the erratum easy to notice.

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