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

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

@options enable to keep params when switching ref.

Fixed a nginx configuration

update links to be relative paths and add CONTRIBUTING.md link

fixed fact about memory usage

fixed capitalization and added Google Analytics

Configure git global settings in installation docs