Fix inheritance of stored_attributes (fixes #14672)

[Brad Bennett, Jessica Yao, & Lakshmi Parthasarathy]
Merge pull request #14833 from jyao6/attribute_inheritance

Fixed Attribute Inheritance Issue
Merge remote-tracking branch 'apotonick/simplify-prefixes'

This is the rebased version of #15026

Closes #15026
Improve CHANGELOG entry
._prefix is private API so we should not recommend to override it
Mark _prefix as nodoc

These methods are private API.
Follow the documentation guideline
Refactoring the code to make consitional return explicit
Following documentation guideline
We need an explicit return

If we don't return early Ruby will memoize the value of the prefix of
the parent class what will make the subsequent searchs to not work as
expected.

If the early return we are avoiding the memoization.

But when using the deprecated path we need to memoize the value, so we
are not using early return for the deprecated path.
Update Redcarpet to 3.1.2

This version fixes an API breakage between 3.0 and 3.1 ; the header
method's arity should not have changed.

[ci skip]
[ci skip]add updated information, ref[#522c0fd]
Merge pull request #15096 from robin850/patch-20

Update Redcarpet to 3.1.2
Merge pull request #15108 from arthurnn/i18n_master

Use i18n master to run tests
Merge pull request #15074 from kuldeepaggarwal/docs-changes

Add missing information, ref[#522c0fd]
remove warning `ambiguous first argument; put parentheses or even spaces`
Merge pull request #15110 from kuldeepaggarwal/fix-warning

remove warning `ambiguous first argument; put parentheses or even spaces`
Fixed a problem where `sum`, `size`, `average`, `minimum` and `maximum` used
with a grouping was not returning a Hash.
rake, remove frontbase related tasks from Active Record Rakefile.
Merge pull request #15070 from ayamomiji/sse-patch

Add multiple lines message support for SSE module