Error "There is no ID mapped for the user key" loading notifications recipients

Original issue: https://product-fabric.atlassian.net/browse/NP-75

This is happening for 2 sites, which causes 500 responses to be returned from /rest/product-fabric/1.0/notifiedusers. The offending users should probably be removed.

https://splunk.atlassian.io/en-US/app/search/search?q=search%20eventtype%3Dmicros_jira*%20err.stack%3D%22*Recipient*%22%20%22*There%20is%20no%20ID%20mapped%20for%20the%20user%20key*%22&display.page.search.mode=smart&dispatch.sample_ratio=1&earliest=-24h%40h&latest=now&sid=1524042539.200788_E7788A4C-2494-4763-81E0-36C703BBF35D

java.lang.RuntimeException: There is no ID mapped for the user key 'scottadamson '. Listener: com.atlassian.jira.event.listeners.mail.MailListener event: com.atlassian.jira.event.issue.DefaultIssueEventManager$$Lambda$5642/1274040015 at com.atlassian.event.internal.SingleParameterMethodListenerInvoker.invoke(SingleParameterMethodListenerInvoker.java:55) at com.atlassian.jira.event.JiraEventRunnableFactory.lambda$null$1(JiraEventRunnableFactory.java:67) at com.atlassian.workcontext.metrics.Timer.time(Timer.java:108) at com.atlassian.jira.event.JiraEventRunnableFactory.lambda$invokeEvent$2(JiraEventRunnableFactory.java:67) at com.atlassian.jira.event.metrics.CodeTimer.runImpl(CodeTimer.java:76) at com.atlassian.jira.event.metrics.CodeTimer.execute(CodeTimer.java:62) at com.atlassian.jira.event.JiraEventRunnableFactory.lambda$getRunnable$0(JiraEventRunnableFactory.java:41) at com.google.common.util.concurrent.MoreExecutors$DirectExecutorService.execute(MoreExecutors.java:299) at com.atlassian.event.internal.AsynchronousAbleEventDispatcher.dispatch(AsynchronousAbleEventDispatcher.java:105) at com.atlassian.jira.event.JiraEventDispatcher.dispatch(JiraEventDispatcher.java:77) at com.atlassian.event.internal.EventPublisherImpl.invokeListeners(EventPublisherImpl.java:98) at com.atlassian.event.internal.EventPublisherImpl.publish(EventPublisherImpl.java:82) at com.atlassian.jira.event.JiraEventPublisher.lambda$publishEvent$0(JiraEventPublisher.java:43) at com.atlassian.jira.event.metrics.CodeTimer.runImpl(CodeTimer.java:76) at com.atlassian.jira.event.metrics.CodeTimer.execute(CodeTimer.java:62) at com.atlassian.jira.event.JiraEventPublisher.publish(JiraEventPublisher.java:39) at com.atlassian.jira.event.issue.txnaware.TxnAwareEventFactoryImpl.lambda$publishEventOnCommit$0(TxnAwareEventFactoryImpl.java:44) at com.atlassian.jira.transaction.RunnablesQueueImpl.lambda$wrapRunnableWithThreadCheck$0(RunnablesQueueImpl.java:48) at com.atlassian.ozymandias.SafePluginPointAccess.runnable(SafePluginPointAccess.java:201)