Migrate `knowledge.db` to `tribler.db` #2303

Job	Run time
changes	6s
env	1s
gatekeeper	4s
documentation	0s
guitest_nix	0s
mac	0s
pytest	0s
ubuntu	0s
windows	0s
guitest / run (windows-latest)	2m 58s
scripttest / run	1m 13s
coverage / generate and upload	26s
	4m 48s

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