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* Introduce a new ReadyIndicator interface that allows waiting for the
  link index to become ready while getting a progress percentage.
* In the BackLinkUpdaterListener, wait for the index to become ready
  when a job is active and display the indexing progress.
* Provide a ready indicator including indexing progress in the Solr
  indexer.
* Add some tests.

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* Remove leftover LinkIndexingStatus.

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* Fix some math problems in the progress calculation.
* Don't directly inject the link store into the link update listener to
  avoid early initialization.

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* Use Future<Void> for the ready indicator as there is no result value.

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* Move the ReadyIndicator to the store API.

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* Separate the SolrIndexerReadyIndicator from the DefaultSolrIndexer and
  add tests.
* Restore the original coverage.

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* Fix the logic for updating progress steps.

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* Modernize the jobRunner JavaScript code
* Continue polling the job status when the job is waiting to detect when
  a question is answered in the background (by another browser tab or on
  the server).

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* Add support in rename and delete job requests to indicate if the
  job should wait for indexing to finish.
* Ask the user after 10 seconds if the refactoring should wait for link
  indexing to finish.
* Add unit tests.

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* Add an integration test.

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* Update since-versions from 16.8.0RC1 to 16.8.0.