[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.