Create a celery task for unindexing from the search engine layers which are linked to an issue

[capooti]

Any layer which will be associated to an issue should be removed from Solr/ES.

A periodic task in Celery should take care of doing this cleanup.

[capooti]

This was closed by #324

Though we would need a test proving it: @thaorell can you take care of this?

[capooti]

What the test need to do:

• add an issue to a service
• add an issue to one or more layers
• run the task
• checks if the index are away from Solr/ES

[capooti]

@dkakkar @bglewis @thaorell now that we have issues, which can be linked to a given service or layer, we need to implement this to have the search engine updated accordingly.

[thaorell]

@capooti Which issues are you addressing?

[capooti]

@thaorell this is linked to #245 (which is implemented and closed now)