Add a selfoss CLI tool

[jtojnar]

This introduces two commands for dumping and loading the database contents:

• php bin/selfoss db:export path.json
• php bin/selfoss db:import path.json

Those are useful for testing, as well as for moving data from one database backend to another.

[jtojnar]

Currently, datetimes exported from SQLite cannot be imported to MySQL, we need some normalization.

[netlify[bot]]

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