[Server support] - Prepare repository code restructure

[matuella]

Moves all files to a monorepo structure, which should be something like:

root
|_ flutter
  - architecture, respective readme, root for flutter project files.
|_ firebase
  - architecture, respective readme, root for firebase project files.
- Readmes, contributing, github, vscode, etc.

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