captcha 8.x-1.0-alpha1

Issue #2486113 by keshav.k, m1r1k, naveenvalecha: Replace md5 with sha256 hashes
Issue #2798991 by naveenvalecha: Fix coding standards & replace the deprecated methods
Issue #2631636 by elachlan, Prashant.c, harsha012: Improve README.md file for CAPTCHA
Issue #2798991 by naveenvalecha: Fix coding standards & replace the deprecated methods
Issue #2668134 by TMontana93: Installation message unnecessary
Issue #2627904 by ddrozdik: Convert hook_help() from 7 to 8 version
Issue #2748859 by fornigam: Need to improve readme file
Issue #2636772 by jamin_melville: Doesn'\''t install as part of distro
Issue #2768689 by Nikhilesh Gupta: Removed deprecated method
Issue #2782479 by chaitanya17, elachlan: Replace deprecated usage of \Drupal::url with Url::fromRoute
Issue #2768705 by Nikhilesh Gupta: Deprecated Method
Issue #2782479 by chaitanya17: Replace deprecated usage of \Drupal::url with Url::fromRoute
Issue #1395184 by Berdir, mcotelo, Arla, cosmicdreams, wodenx, mpdonadio, vimalgoradiya, valery86, elvis2: Forms with AJAX trigger "CAPTCHA session reuse attack detected." error
Issue #2783351 by kerby70: Cron test - Verify old captcha session are removed and a new one is not
Issue #2773767 by ziomizar: deprecated function db_delete
Issue #2754191 by rajeshwari10: Remove @file tag docblock from all the .php files
Issue #2707509 by mkdok: Can't see Image CAPTCHA image unless enable Node permission: [x] View published content