fix(replays): fix bin/mock-replays script

[michellewzhang]

Closes getsentry/team-replay#115

bin/mock-replays script now runs successfully locally so you can see an empty replay in your local environment upon running sentry devserver

[michellewzhang]

Some future work that needs to be added upon this fix:

Priority TODOs

• Alert the user if the replay feature flag is not on (this needs to be added to your local sentry.conf.py file)
• Make the created project a Javascript project by default (currently, trying to switch it to Javascript gives an error)
• Assign the project to the default team (currently, not assigned by default)

Nice to have TODOs

• Replay video contains rage clicks and dead clicks (and isn't just an empty video)

[JoshFerge]

nice, great stuff! 🥳