[US23] Local database to cache repositories' data

[loggerz]

The app should be usable even without Internet connectivity using the data previously downloaded. As a first step only implement the logic related to read-operations.

Acceptance criteria:

• all read operations from repositories are performed locally
• a refresh operation is available to pull the most up-to-date version of the data
• the repositories API must be modified as little as possible
• the data is stored using either a standard feature of Firebase or the local SQLite feature of Android

[loggerz]

Update The Firebase offline API is way more extensive than anticipated : this can implemented by the simple removal of one line of code during sign-in. Few edge cases must be taken care of but otherwise, this feature is already enabled for the Firestore and only the document storage must be implemented on our side