Database/cache user interest

[Huntsekkern]

Closes #216

Pending merging #241.

In the end, even though the feature is used nowhere, with the cache implementation I made, local user can still benefit from the cache at no extra-cost. And on top of that, in a dream version, we could even ID our local users on our database even though they're not logged in.

Pull reviewers stats

Stats for the last 180 days:		User	Total reviews	Median time to review	Total comments
	Huntsekkern	34	11h 54m))	65
	Snowflake8	30	7h 2m))	61
	so7fie	29	13h 38m))	60
	RaulinN	36	9h 20m))	56
	DGoedtkindtEPFL	42	15h 38m))	56
	Laniakea1999	22	12h 8m))	15

Pull reviewers stats

Stats for the last 180 days:		User	Total reviews	Median time to review	Total comments
	Huntsekkern	34	11h 54m))	65
	Snowflake8	30	7h 2m))	61
	so7fie	29	13h 38m))	60
	RaulinN	36	9h 20m))	56
	DGoedtkindtEPFL	42	15h 38m))	56
	Laniakea1999	22	12h 8m))	15