* initial implementation of after, before, on search flags allowing to restrict the search to a specific day or a date range
* missed setting beforeDate in SearchParams in one place
* fixed condition when only flags are used for search without any plain terms
* changed date format used for after/before/on flags to be in ISO8601 format as suggested in PR comments, added a helper function to pad month and day with zeroes allowing the user user either format, with or without leading zeroes
* corrected expected compare to date setting for the TestParseDateFilterToTimeISO8601 test
* fixed a bug for the scenario when you only have the date flags without any terms, added a couple of tests for that scenario
* updated the date filter logic to use parameters to construct the query instead of simply appending strings together, as suggested in the pull request comments
* added search unit test using date flags
* added a helper function to create a test post with a createat date manually set, updated the test for search using date flags to create test posts with different createat dates to be able to better test the functionality
* MM-11817 Add support for after/before/on search flags with Elasticsearch
* add support to search posts to perform the search in context of the client's timezone when filtering by createat date using on: after: before: flags
* updated tests to match the new signature
