Migrate from `master` to `main` branch

Use of the master and slave terms in the computer industry is widespread, but has been recently discouraged, as it was brought to attention that the terms may be considered offensive. GitHub has already moved their default repositories from the master to main in many places. We're planning to do the same for our repositories, this however requires some changes in the code. After the proposed changes get merged to the master branch, the repository should be ready for the migration from master to main.

To migrate from master to main:

Do a local branch rename with git branch -m master main
Push the renamed branch git push -u origin main
In the Github repo settings, rename the default branch: Settings -> Branches -> Default branch -> (pencil icon) -> (change master to main) -> (approve)

After migration other contributors should update their local repositories:

git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a

keep-network / tbtc-dapp

Migrate from `master` to `main` branch #399