Add support for review apps for site contributions

[starbelly]

Currently if someone, including myself does a PR to the website there is no clear visibility into how the changes look, whether they work, etc. Instead it requires the reviewer to pull down the contributors branch, build assets, etc. etc. in order to effectively review a change.

We should use gigalixir (free tier surely) for spinning up review apps. This is relatively simple https://gigalixir.readthedocs.io/en/latest/deploy.html#how-to-set-up-review-apps-feature-branch-apps. Doing this in github actions is essentially the same, gigalixir made a howto using github actions but can't seem to find it ATM.

[starbelly]

Closing this in favor of erlef/website#317