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.