[skip ci] Reorder i18n guide

Currently, the section called "How to store your custom translations"
has several subheadings that make no sense, such as "Translations for
ActiveRecord models." These make more sense under the "Overview of the
I18n API Features" section. I moved the "How to store..." section down
to the more appropriate sub-headings "Using Different Backends" and
"Using Different Exception Handlers" and removed the "Customize your
i18n setup" header.
[ci skip] Added examples for color_field_tag method
Merge pull request #14771 from heironimus/i18n_guide_reorder

Reorder i18n guide
Merge pull request #14779 from zenati/master

[ci skip] Added examples for color_field_tag method
[ci skip] Added examples for search_field_tag method
Merge pull request #14780 from zenati/master

[ci skip] Added examples for search_field_tag method
[ci skip] Added examples for telephone_field_tag method
[ci skip] Added examples for url_field_tag method
Merge pull request #14781 from zenati/master

[ci skip] Added examples for telephone_field_tag and url_field_tag methods
Autoload ActiveModel::StrictValidationFailed

Currently, if environment doesn’t eager load code, invoking this
constant before calling #valid? on a model instance results in a
NameError.
Merge pull request #14783 from hakanensari/autoload-strict-validation-failed

Autoload ActiveModel::StrictValidationFailed