Update default branch name from "master" to "main"

[mmmavis]

• Default branch of this repo should be renamed from master to main.
• Make sure if there's anything we need to change on Heroku side

[mmmavis]

@cdanfon Could you help triage this, please? Thank you!

[cdanfon]

Hey @mtdenton this one could be looked at in a Maintenance Fridays™ (but because is in the pulse repo I can't add it to the Q4 Maintenance Milestone).

Could you prioritise in the ZenHub backlog please? Ta