Add Hex package configuration and display "Find on Hex" footer links

[wojtekmach]

Demo:

Screen.Recording.2021-08-12.at.11.57.42.mov

Future work: (out of the scope of this PR)

1. Support Hex diff. Currently when linking to Hex diff, we need to include two versions to diff against. It would be nice to be able to provide just a single version, e.g.: https://diff.hex.pm/diff/decimal/2.0.0, and Hex diff would automatically pick the version immediately preceding that.
2. Support Hex private packages. We can already do that for packages themselves but let's wait for private packages support in Preview (Support Hex private packages hexpm/preview#48) and possibly Diff (Support Hex private packages hexpm/diff#74)

[wojtekmach]

@ericmj in the pr description I mentioned future work, any feedback on that in particular would be appreciated :)

[ericmj]

1. Sounds good, please open an issue for that.
2. That is on the roadmap but currently low priority personally.

[wojtekmach]

Created hexpm/diff#75 for number 1, thanks for the feedback!