Add true persistence for stored data

[jagoe]

Currently, BabelBot stores data in memory and will lose it after a restart.
Instead, BabelBot should store data persistently.

• There should be a storage default that doesn't rely on external databases.
• BabelBot administrators should be able to point it to an existing database instance.
• The Helm chart should reflect those options and provide a default that doesn't need any configuration.

[RauchF]

Good point. I've added this issue to the Multi-Node Support milestone as I believe it's more or less essential for that too :)