Add Firebase emulators

[th0rgall]

• [x] Test that the local demo environment works
• [ ] Test that the wtmg-dev environment works, hooked up to all emulators
• [x] Test that the wtmg-dev environment works, hooked up only to the function emulators
• [x] Test that the live wtmg-dev environment works

Fixes #288

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit d3d5305):

https://wtmg-dev--pr293-firebase-emulators-ayhgag35.web.app

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[th0rgall]

With the last commit https://github.com/WelcometoMyGarden/welcometomygarden/pull/293/commits/d3d530547d9a48a7b03c6ce496574bc718f9ccb3 added, this hopefully also fixes #98.