[Server support] - Create firestore security rules, indexes and tests to support new DB model

[matuella]

Using this model as reference - and after https://github.com/olmps/memo/issues/137 is done - we should create a proper firestore.rules and firestore.indexes.json which correctly secures the application at an individual (authorized operations) and global (unauthorized) scope.

These rules should be 100% unit tested, using a local firestore emulator instance to run such environment.

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Sub-issue that relates to the Server support - https://github.com/olmps/memo/issues/136.