[MM-22037] Enable uppercase letters in the email (#13794)

* Enable uppercase letters in the email

* Lowercase email on every input

* Remove invalid test

app/import_functions.go

@@ -309,6 +309,7 @@ func (a *App) importUser(data *UserImportData, dryRun bool) *model.AppError {
 		hasUserChanged = true
 		hasUserEmailVerifiedChanged = true // Changing the email resets email verified to false by default.
 		user.Email = *data.Email
+		user.Email = strings.ToLower(user.Email)
 	}
 
 	var password string

app/slackimport.go

@@ -185,6 +185,7 @@ func (a *App) SlackAddUsers(teamId string, slackusers []SlackUser, importerLog *
 			continue
 		}
 
+		email = strings.ToLower(email)
 		newUser := model.User{
 			Username:  sUser.Username,
 			FirstName: firstName,

app/user.go

@@ -1611,6 +1611,7 @@ func (a *App) VerifyEmailFromToken(userSuppliedTokenString string) *model.AppErr
 		return err
 	}
 
+	tokenData.Email = strings.ToLower(tokenData.Email)
 	if err := a.VerifyUserEmail(tokenData.UserId, tokenData.Email); err != nil {
 		return err
 	}