Add "copy link to file" anchor for each file diff

[evangrim]

Feature Request

Generate anchors for each file diff, and a button to copy URL with diff, for instance https://react-native-community.github.io/upgrade-helper/?from=0.62.2&to=0.68.4#metro.config.js

Why it is needed

Throughout upgrading, I need to make decisions based on educated guesses (I'm not really an expert in the ecosystem). There are a lot of loose ends to follow up on -- this or that outdated package, various hacks involved in installing them, etc.

Being able to easily copy a reference to a specific file diff would help me provide context for such notes.

Possible implementation

Each file diff (header or container) has an ID based on its path. Add "Link" icon next to "Copy file path" using same clipboard mechanism.

Code sample

[mjstelly]

@pvinis This functionality currently exists. It can be closed.