Collapse where constraints to one where constraint

In order to remove duplication with joining arel where constraints with
`AND`, all constraints on `build_arel` are collapsed into one head node: `Arel::Nodes::And`

Closes: #11963
[ci skip] Update plugins.md

Highlighted code.
Merge pull request #12231 from harshadsabne/master

[ci skip] Update plugins.md
[ci skip] Consistency wording of 9.6 in form_helpers.md

Chapter 9 is giving a user and set of addresses example.
So 'Add a new address' would be more consistent.
Also change the javascript to JavaScript.
Merge pull request #12239 from JuanitoFatas/form_helpers

[ci skip] Consistency wording of 9.6 in form_helpers.md
Removes unused code related to DatabaseTasks.
Merge pull request #12245 from kennyj/remove_no_using_code_20130916

Removes unused code related to DatabaseTasks.
[ci skip] Improve readability of 4.3's NOTE in migration.md.
Merge pull request #12011 from jetthoughts/11963_fix_join_with_association_scope

Collapse where constraints to the Arel::Nodes::And node

Conflicts:
	activerecord/CHANGELOG.md
Merge pull request #12250 from JuanitoFatas/migration

[ci skip] Improve readability of 4.3's NOTE in migration.md.
Remove tzinfo dependency from Action Pack

This gem is used by Active Support but it should not be a dependency of
Action Pack.
Merge pull request #12262 from sferik/remove_tzinfo_from_actionpack

Remove tzinfo dependency from Action Pack
There's no need to do this

AS does the following inside Time.find_zone! ...
`ActiveSupport::TimeZone[time_zone] || TZInfo::Timezone.get(time_zone)`
and given that the test is stubbing AS::TZ[] we don't need the removed
code.
Fix an issue where router can't recognize downcased url encoding path.
Merge pull request #12276 from kennyj/fix_12269

Fix an issue where router can't recognize downcased url encoding path.
do what the superclass does in the case that objects do not match
ActiveRecord::Base#<=> has been removed.  Primary keys may not be in order,
or even be numbers, so sorting by id doesn't make sense.  Please use `sort_by`
and specify the attribute you wish to sort with.  For example, change:

  Post.all.to_a.sort

to:

  Post.all.to_a.sort_by(&:id)
support objects with blank string primary keys
remove unused variable
NullSessionHash#destroy should be a no-op

Previously it was raising a NilException