Issues bulk update

A little change on wording for new-user email

better wording encouraging usage of latest stable release in GitLab Maintenance Policy

Dont parse gfm-extractions as commit. Increase gfm rendering speed. Especially for a lot of code snippets

adds missing wording missed in https://github.com/gitlabhq/gitlabhq/commit/58d6176bc35fb45427df75b9762df6bb0d3b3a98 and https://github.com/gitlabhq/gitlabhq/pull/3902

Update README.md

Issue 3941: Code preview dark doesn't display correctly with long lines

fixes #3115 and adds support for gif bmp & tiff images

when showing image preview check via lowercased extension and adds support for gif bmp & tiff images

Assigning the background color to the pre tag did not work correctly. Moved background color to the highlight class which fixes the problem.

Adding PostgreSQL to README as an alternative requirement

Fix bubble padding.

Config setting should be used in git version check.

fix #3823