Make remote_ip detection properly handle private IPv6 addresses

Fixes #12638.
SQLite3: Always close statements.

SQLite3 adapter must make sure to close statements after queries.

Fixes: #13631
Update the Getting Started tutorial

Update the Getting Started tutorial to clarify where and how to add
controller actions, and fix some grammar.

This PR is an improved version of #14828, #14822 and #14815.

[skip ci]
Merge pull request #14905 from jonatack/patch-7

Update the Getting Started tutorial
rewrites the API docs of favicon_link_tag [ci skip]

Motivated by #14907.
the use of ... in place >> using ... instead

[ci skip]
Merge pull request #14909 from jonatack/patch-7

the use of ... in place >> using ... instead
split nil / Hash cases in url_for

this reduces the number of comparisons and method calls `url_for`
requires.  The nil case no longer calls `symbolize_keys`, we already
know options is nil, so no more ||=, and since it is nil we already know
that options[:host] will be nil too.
Stringify variables names for mysql connections

For mysql2/mysql adapters, `sql_mode` variable name set in `database.yml`
as string, was ignored and `sql_mode` was set to use strict mode.

Fixes #14895
don't allocate string on hash access
do not allocate strings while creating urls
eliminate conditional when sending the named route method
avoid calling extract_record multiple times
[skip ci] Document: required `via` option in `match` routing method.
Add documentation to select_tag for :selected option
Give real privacy to class methods in AR::PredicateBuilder
Merge pull request #14916 from moktin/real_privacy_for_functions_in_predicate_builder

Give real privacy to class methods in AR::PredicateBuilder
Merge pull request #14922 from rizwanreza/add-selected-documentation-to-select-tag

Add documentation to select_tag for :selected option
Avoid unnecessary conditional block

GeneratedAttribute#column_name checks already for reference? on the
attribute returning the right name to use as symbol for label and field.
Merge pull request #14928 from potomak/update-scaffold-form-template

Avoid unnecessary conditional block