use flat_map
Updated the guide to say that the localisation file for validation messages is in activemodel, not activerecord
Add "extname" option to javascript_include_tag

ActionView::Helpers.asset_path is where the logic for
javascript_include_tag resides.  It takes an extname option for
specifying the extension or false to not append it.  This exposes that
option to javascript_include_tag.

Without the option files that didn't end with ".js" would get the
extension appended to them.  This broke JST templates and other file
types that should be interpreted as JavaScript but who's file extension
isn't ".js"
[Fixes #11734]Correct Association Basics Guide on has_and_belongs_to_many[ci skip]

Done the following changes:
 1. Replace uniq call in example with autosave as uniq is not supported
 on has_and_belongs_to_many
 2. Add :readonly option to list of supported call as per
 [EdgeApi](http://edgeapi.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_and_belongs_to_many)
Changing the def self.table_name to self.table_name in the column_name.rb
fix for postgresql failing test in local
used flat_map instead of map.flatten in /guides/rails_guides/helpers.rb
Merge pull request #11730 from neerajdotname/use_flat_map

use flat_map
Merge pull request #11741 from Karunakar/failing_test_in_local

fix for postgresql failing test in local
Merge pull request #11707 from nathanstitt/extname_option

javascript_include_tag "extname" option for use by asset_path
Merge pull request #11744 from Karunakar/minor_fix_on_flatmap

used flat_map instead of map.flatten in /guides/rails_guides/helpers.rb
Removed an unnecessary loop - it kills performance on large texts.
Merge pull request #11740 from Karunakar/self_table

Changing the def self.table_name to self.table_name in the column_name.r...
Merge pull request #11738 from gaurish/guides-association-basics-fix

[Fixes #11734]Correct Association Basics Guide on has_and_belongs_to_many
Merge pull request #11600 from jasiek/master

Removed an unnecessary loop - it kills performance on large texts.
Add note that you must be in the rails app directory when starting up the server.

This is a response to issue #11731 and is a common mistake for people
newer to rails that try forget to cd into the new rails app after
running rails new.
Merge pull request #11733 from iainbeeston/master

i18n guide update: corrected reference to activerecord to instead reference activemodel
Merge pull request #11747 from bradly/guides_getting_started_fix

Add note that you must be in the rails app when starting the server
No need to abbreviate application

[ci skip]
:scissors:

[ci skip]