UserStore migration (#15563)

* Migration completed

* Fix tests

* Fix tests

* Fix tests

* Suggestions

* Trigger CI

* Suggestions

* Merge with master

* Trigger CI

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>

app/constants.go

@@ -4,3 +4,5 @@
 package app
 
 const MISSING_CHANNEL_MEMBER_ERROR = "app.channel.get_member.missing.app_error"
+const MISSING_ACCOUNT_ERROR = "app.user.missing_account.const"
+const MISSING_AUTH_ACCOUNT_ERROR = "app.user.get_by_auth.missing_account.app_error"