Backfill changelog

[rileyhgrant]

Resolves: #128

Preview link for convenience: https://gnomad.broadinstitute.org/news/preview/127/changelog

Adds entries to the changelog that were not catalogued.

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The criteria for adding something to the changelog for this PR was:

• user facing changes (re-ordering of links, changing referenced papers, bug fixes, functionality changes)
• major non-user facing changes (updating a language, major version update to database)

As such, these changelog additions do not include

• minor technical fixes (refactoring of bad design patterns)
• minor copy/content fixes (updating an old checksum for a download)

Let me know if this criteria should change, and changelog posts can be added or removed.

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The changelog has been neglected for many months now (my bad). General practice surrounding adding to the changelog as browser changes happen will improve going forward.

[rileyhgrant]

I realize this is a lot of copy to be reviewed in a single PR. I'd guess that the view commit by commit feature could be helpful, but also I'd think that since every changelog is a seperate .md file, GitHub's default review view will be clear.

If anyone prefers that these entries be split into multiple PRs, say the word.