Switches deployment to Github Actions, which includes IPFS build and publish, as well as DNS changes. Fixes #446.

By design, will be deployable from PRs, as well as develop+master branches:
PR builds -> pr-{number}.yearn.dev.
Develop push -> yearn.dev.
Master push -> yearn.finance.

Sample PR available here, which highlights deployment steps and output: https://github.com/B3RX8OIPDHDT3/yearn-finance-v3/pull/2
Sample build available from here: https://pr-2.b3tools.xyz

Issues/considerations:

1. Currently IPFS build is being pushed to Pinata Cloud, is that the preferred platform for IPFS builds?
2. This workflow requires DNS updates to point to the latest IPFS cid hash. I've used Route 53 to test out DNS changes(which yearn uses I believe), and it requires two records - CNAME pointing to a gateway, as well as TXT record for DNSlink. Route 53 can set CNAME for subdomains, but it does not allow setting CNAME for a domain as per DNS spec, which means it wouldn't be able to deploy deploy(yearn.dev) and master(yearn.finance).

There are several workarounds for this, probs missing some obvious ones as well:

• Using Cloudflare for DNS since it allows setting CNAME at the apex domain level, and would work well with IPFS?
• Possibly pointing record to Cloudfront, but need to investigate how exactly it would work.

3. Should API keys be in secrets when in production, would assume yes? Current values for REACT_APP_* are taken from .env.example, which will need changing then.
4. Cleanup job will need to be created, which would remove DNS records + IPFS pin when PR is closed and branch is removed.