Move tag checking logic to User store

Created by: asdine

This PR reorganizes duplicated tag verification logic by moving it to the User store.

Created by: sourcegraph-bot

Notifying subscribers in CODENOTIFY files for diff 9e1f71114ee7efcd461a28118520ac31f79c0215...fafc2c0b3a97dc946450403220c4e4218b826941.

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Created by: codecov[bot]

Codecov Report

Merging #16166 (fafc2c0) into main (9e1f711) will increase coverage by 0.00%. The diff coverage is 76.19%.

@@           Coverage Diff           @@
##             main   #16166   +/-   ##
=======================================
  Coverage   52.77%   52.78%           
=======================================
  Files        1646     1646           
  Lines       82283    82282    -1     
  Branches     7334     7334           
=======================================
+ Hits        43427    43434    +7     
+ Misses      35004    34995    -9     
- Partials     3852     3853    +1     

Flag	Coverage Δ		*Carryforward flag
go	52.57% <76.19%> (+0.01%)
integration	28.42% <ø> (ø)		Carriedforward from 1232cc5
storybook	28.03% <ø> (ø)		Carriedforward from 1232cc5
typescript	53.30% <ø> (ø)		Carriedforward from 1232cc5
unit	35.88% <ø> (ø)		Carriedforward from 1232cc5

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
cmd/frontend/backend/users.go	0.00% <ø> (ø)
internal/db/users_mock.go	0.00% <ø> (ø)
cmd/repo-updater/repos/syncer.go	76.13% <66.66%> (-0.26%)
internal/db/users.go	47.49% <72.72%> (+0.75%)
cmd/frontend/graphqlbackend/external_services.go	68.63% <85.71%> (+0.56%)

Created by: asdine

@tsenart @ryanslade I have moved the code to the db.Users store. The CheckActorHasTag function has been removed as it's only used once and getting the actor is a simple operation, so it made sense to remove it. PTAL