[Feature] Add Databases/Source as new Table/Datatype in Database

Originially proposed in thg-tuda/bp/greenhouse-gas-balancing-tuda-frontend#47

Summary

We could introduce a new Table/Datatype for Sources. A single source then could have:

1. A name
2. A List of Versions
3. General Comments
4. URL
5. (and later, not MVP: An API Connector if possible)

The emission factors could then be extended by the following:

• A relation to the database
• A relation to the database's version
• Original name in the database of origin

What is the added value of this feature

• Reusability of sources
• Versioning of emissionfactors by database source versions
• Sorting and grouping/ querying by databases

UI-Mockups, Prototypes, Vision

(Paste screenshots of a ui-mockup here, if applicable. Describe your vision. Demonstrate a Prototype)