Use `main` as the default branch

### Background
The default branch for https://github.com/dbt-labs/hub.getdbt.com is currently `master`. It is hard-coded within Python in a place or two within this repo (https://github.com/dbt-labs/hubcap). Use `git grep "master" ./` to discover those locations. It is unknown if this branch name is also specified within Netlify.

### Next steps
1. Discover if the branch name is hard-coded within Netlify configuration -- update the following steps depending on the discoveries
2. Copy `master` to `main` within https://github.com/dbt-labs/hub.getdbt.com
3. Update hubcap to point to `main` branch
4. Redploy hubcap
5. Drop `master` branch in hub.getdbt.com