Deduplicate tags

[jonafato]

In the course of upgrading Pelican from 3.7.1 to 4.0.1, I found that we have duplicate tags that now interact with Pelican's content discovery logic in a way that introduces different output across builds (e.g. a tag's representation might be "Data Science", "data science", or "data-science", all pointing to the same tag slug URL). The changes that would resolve this issue are:

• Tags should be consolidated into canonical representations (e.g. "Data Science")
• A data test should be added to ensure that new duplicate tags (as defined by tags that resolve to the same slug using Pelican's slugify function) are not added in future data contributions
• Redirects should be added to the redirects.txt file where applicable
• When the above are complete, we should be able to remove the legacy add_files method from the PyVideoArticleGenerator class