Filter enhancement: Filter issues on when they were created or closed
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### User story
A project manager would like to see all issues with a given label that were created or closed between two arbitrary dates.
### Proposal
Add search filters `created_after`, `created_before`, `closed_after`, and `closed_before` that takes a specific date such as `2017-04-12` or a relative date such as `2 days ago`, as argument.
### Links / references
Google Code had this back in the days. GitHub also has this feature: https://help.github.com/articles/searching-issues/#search-based-on-when-an-issue-or-pull-request-was-closed
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