Support migrating of data from one installation to another

[range-of-motion]

This discussion was brought up in the Discord server.

Before I go on, let me be clear that I intend to keep https://app.budgethq.com alive until the end of times.

TLDR–what happens if the hosted instance (https://app.budgethq.com) stops existing? For example, if I die tomorrow, and my balance on DigitalOcean runs out, is there any way to retrieve my data (transactions, tags, etc.) and use it on another installation (that you might host yourself).

There is no way to do that right now, but I think it's something worth implementing. Hence this issue.