Data dump

[muety]

Option for users to download a big, fat dump (probably zipped CSVs) of all their data to easily migrate from one instance to another, e.g. from Wakapi.dev to self-hosted or vice versa.

Still undecided, if we want restore functionality as well (i.e. upload and import the dump again) or if it would be sufficient to add precise documentation on how to import these dumps directly to the database.

Potentially relates to #321, #323.

[muety]

MySQL: https://dev.mysql.com/doc/refman/8.0/en/select-into.html

Postgres: https://www.postgresql.org/docs/current/sql-copy.html

SQLite: Looks like there is no way to directly dump a query into a file without using the CLI

[muety]

Decided to use Wakapi's already existing import functionality for this.