Use symbols instead of strings

ActiveSupport::Concern is used all over Rails
This PR will only create 3 new objects as keys are never recreated and
are not subject to garbage collection.

The strings were being uniquely created and garbage collected. I don't
have any performance numbers but this should be better than all of the
GC.
Improve js written for guides

* Stop using unnecessary global variable
* Use function defined by jQuery for simplicity
* Fix event listner for "more info button" to handle
  event on resize browser
Making the rake file for tests easier to read.

I'm defining a new class which modularizes how the `rake test` tasks are
defined and invoked.
Improved grammar and replaced 'dbs' slang with 'databases'
Add few information about missing steps [ci skip]

Add some deprecations previously note covered such as the confirm
option for the link_to helper or options which aren't needed anymore

Also add a mention about the config.assets.js_compressor option.
cookies are encrypted starting Rails 4
doc for more input tag types
store a symbol directly inside DISPATCH_CACHE for extra roflscale
Do not invoke callbacks when delete_all is called

Method `delete_all` should not be invoking callbacks and this
feature was deprecated in Rails 4.0. This is being removed.
`delete_all` will continue to honor the `:dependent` option. However
if `:dependent` value is `:destroy` then the default deletion
strategy for that collection will be applied.

User can also force a deletion strategy by passing parameter to
`delete_all`. For example you can do `@post.comments.delete_all(:nullify)`
updated AS:Callbacks doc for terminator option in define_callbacks method
The change is commited at ba552764344bc0a3c25b8576ec11f127ceaa16da
Complete a sentence in i18n guide's introduction

Add a list of what will be covered into this guide
remove evals from AM::Validations::Callbacks
follow the same refactor at a63a964a5d1ed02cf0df1b1a33a96ed2a9fa987b
Removed params permit step from 5.6 section [ci skip]
Merge pull request #11196 from yangchenyun/remove_evals_in_am_callbacks

Remove evals from AM::Validations::Callbacks
fix typo in ActiveModel::Error docs [ci skip]
Merge pull request #11192 from charliesome/extra-roflscale

Store a symbol directly inside DISPATCH_CACHE for additional roflscale
Adding find_by in guide to methods that trigger after_find
unified the param names across all callbacks manipulation methods
_ Rename the define_callbacks params to `names`
  - in order to match the naming conventions for `get_callbacks` and `set_callbacks` at https://github.com/rails/rails/blob/master/activesupport/lib/active_support/callbacks.rb#L736-743
  - `define_callbacks` just register names(events), not define the real callback functions.
- Rename the `reset_callbacks` params
Using preferred find_by syntax in guides
Merge pull request #11204 from Gawyn/find_by-triggers-after_find

Adding find_by in guide to methods that trigger after_find