Clean up lemmy_db_schema expression module

[dessalines]

To be less confusing, the new modules should be:

• lemmy_db_schema::expressions (non-tuple expressions)
• lemmy_db_schema::joins (things with .on)
• lemmy_db_schema::selects (things that are meant to be the argument to .select)

The name "queries" is not accurate, so it should not be used.

Currently, there's some things in selects that don't belong there.

Also I think that doing this refactoring in the same PR as changes to views is too messy, so should be avoided in the future. It is however okay to add a new function in a new module, then do a separate PR that moves other things to that new module.

Originally posted by @dullbananas in #5924 (comment)

[dessalines]

@dullbananas I agree that queries isn't accurate, but I would like all these helper functions to be in the same top level mod. Maybe just db_functions ?

I'm not sure exactly which things should be in selects vs expressions.

Lets have this come after #5924 anyway.