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.

Fix bubble padding.

Config setting should be used in git version check.

fix #3823

Save backup time as is.  Fixes #3857.

Fixed puma.rb environment setting

A small update of nginx example config to prevent users from issues while nginx not listen on localhost

Show only selected branch

A little change on wording for new-user email

included more details on searching for issues related to pull request