fix: various issues after ent refactor

[mircearoata]

• check for either the email or oauth ID matching when logging in
  • in production, logging in was checking if either the email or oauth ID were matching an existing user
  • I changed my email default github email since signing up, so checking that both are equal would not allow me to sign in
  • if checking for both being equal, no account linking would be possible, and the unique email address constraint would also error when logging in with the same email from a different oauth provider
• reindexing mod files was endlessly fetching the 101-200 mods, instead of using the current offset
• query.Clone() discards the effects of Modify, so we take the embedded struct instead, which contains the wanted effects of Modify
  • this was also causing the custom last version date ordering to not have any effect
• build custom predicates that contain arguments with ent methods
  • ent uses PREPARE, and postgres doesn't seem to support question mark arguments in those, even though their docs show that as an example
• query tags in usermod.Mod resolver
  • it's queried on the user page, and the tags field was null, even though the GQL schema specifies a non-null array
  • not really a permanent fix, as there are other places that return a Mod but don't query its tags, resulting in a null tags array
• don't return an error in the dataloader when a mod has no versions, or a version has no dependencies
• check and update existing UserMod entries
  • previously, each authr entry in the updateMod attempted to create a new UserMod entry, which errored because of the unique index

[codecov]

Codecov Report

Attention: Patch coverage is 8.33333% with 55 lines in your changes missing coverage. Please review.

Project coverage is 15.96%. Comparing base (65bcbc9) to head (4d8a9fc).
Report is 59 commits behind head on staging.

Files	Patch %	Lines
gql/resolver_mods.go	0.00%	17 Missing ⚠️
db/mod.go	23.52%	13 Missing ⚠️
db/oauth.go	0.00%	5 Missing ⚠️
dataloader/loaders.go	0.00%	4 Missing ⚠️
gql/resolver_guides.go	0.00%	4 Missing ⚠️
gql/resolver_sml_versions.go	0.00%	4 Missing ⚠️
gql/resolver_versions.go	0.00%	4 Missing ⚠️
gql/resolver_tags.go	0.00%	3 Missing ⚠️
gql/resolver_users.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           staging      #75      +/-   ##
===========================================
- Coverage    17.06%   15.96%   -1.11%     
===========================================
  Files          102      234     +132     
  Lines         5374    59523   +54149     
===========================================
+ Hits           917     9502    +8585     
- Misses        4356    47975   +43619     
- Partials       101     2046    +1945     

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