Add cluster reindexing feature to our ES integration

What does this MR do?

This MR automates steps for the cluster reindexing process gitlab-com/gl-infra/production#2213 (closed)

To schedule a reindexing job you will need to run rake gitlab:elastic:reindex_cluster which will create a ReindexingTask record.

ElasticClusterReindexingCronWorker runs every minute and executes a job with in_progress flag. We can only have 1 job with this flag since we have a unique index.

#213629 (closed)

Reindexing logic/stages

After we create a ReindexingTask it has initial stage

• initial stage is needed to complete initial checks (i.e. storage requirements) and to enable indexing pause feature. We expect a delay between stages to be enough for existing indexing requests to complete.
• reindexing stage creates a new index and triggers Elasticsearch reindex API
• in_progress stage is the polling stage, we're waiting for the task to complete
• As final stages we have success and failure

Stages graph visualisation

Database changes

This MR introduces a table called reindexing_tasks and ReindexingTask model

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #213629 (closed)