Enable staging deployment

[Naramsim]

This PR enables a conditional deploy job on CircleCI so that we will have a productive environment and a staging one. This will allow us to check all the APIs and the website in a separate environment, and, if everything seems right, to deploy safely to https://pokeapi.co/.

A more descriptive and general overview is detailed in this PokeAPI/pokeapi#488 PR.

This deploy job will be mostly invoked by API calls done by pokeapi.co and api-data. This job will deploy to the productive Firebase (pokeapi.co) if this repo gets pushed or if the master branch of either pokeapi.co or api-data is pushed. When deploying it will always use pokeapi.co and api-data's artifacts based on their master branches.

When the branch staging of either pokeapi.co or api-data will be pushed then the job will deploy the artifacts based on their aforementioned staging branches to a new Firebase staging environment (https://pokeapi-test-b6137.firebaseapp.com) (I will give access to @phalt, @tdmalone, @cmmartti to this Firebase project). It would be also nice if @phalt could create the subdomain staging.pokeapi.co and link it to the Firebase project. To do so we should create the new domain, add a TXT record type with host equal to pokeapi.co and value equal to google-site-verification=gV4NnTbUxNds2dIATNVlEyHCfWBgOjte2cNuAdlIVA0. Then probably we need to add the Firebase nameservers to the Namecheap domain config. I will also need to change some strings here and there in the PR I just opened.

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This PR is linked to the following PRs:

• Enable staging deployment pokeapi.co#76
• Enable staging deployment api-data#42
• Update data automatically and enable staging deployment pokeapi#488

This PR should be the first to be merged among them.

[sargunv]

LGTM! For the future, it might be worth moving this stuff into GitHub Actions from CircleCI just to consolidate it all into one platform, but that's def not a blocker for this PR.