[db-schema] Flatten ExtensionVersion

[svenefftinge]

The entity model is a little too much normalized and with that inefficient to request.
I suggest we

• inline dependencies and bundledExtensions as text using comman-separated extension identifier (<namespace>/<name>).
• inline engines, categories, tags as text using comma-separated values.

[spoenemann]

comma-separated extension identifier (<namespace>/<name>)

These are written as <namespace>.<name> in package.json, so let's keep that format.

CC @danieldietrich

[brianking]

Fixing the download count (issue #283), a priority, is dependent on this issue.

[amvanbaren]

@danieldietrich solved this in issue #264. This issue can be closed.