Better missing `secret_key_base` error message

Previously the error was:

```
RuntimeError: You must set secret_key_base in your app's config
```

Will now be:


```
RuntimeError: Missing `secret_key_base` for 'production' environment, set this value in `config/secrets.yml`
```
Fix railties tests in master

Tests are failing due to missing env var on master https://travis-ci.org/rails/rails/jobs/15930622#L641

This adds an environment variable `ENV['RAILS_SECRET_KEY_BASE']` so these tests will pass.
Merge pull request #13469 from schneems/schneems/no-database-errors-all

Raise NoDatabaseError when db does not exist
Merge pull request #13471 from schneems/schneems/better-secrets-error-message

Better missing `secret_key_base` error message
Merge pull request #13472 from schneems/schneems/fix-master-database-url-tests

Fix railties tests in master
Revert "Ensure secret_key_base is set for all environments"

A better solution has been pushed to master.

This reverts commit 959cfcef7255bba720ce3f15323056533ea7b50a.
Move mysql2 test for when adapter will be loaded

When run with only the Mysql adapter, we get this failure: https://travis-ci.org/rails/rails/jobs/15937907#L2416

Porting the test over to only run when mysql2 is loaded
Unused class in AV test
Unused classes in AP tests
using symbol instead of string in establish_connection
Merge pull request #13480 from kuldeepaggarwal/f-removed-depricated-config

using symbol instead of string in establish_connection
Typo rectified commom => common[ci skip]
Merge pull request #13482 from aayushkhandelwal11/typo

Typo rectified commom => common[ci skip]
Added Backslashes to ActiveModel::AttributeMethods to prevent unwanted links in the rdoc + some other doc fixes.[ci skip]
Test with 2.1.0
Merge pull request #13478 from schneems/schneems/fix-mysql

Move mysql2 test for when adapter will be loaded
Avoid getting redefined method warning

Warning:

  ~/projects/rails/activerecord/lib/active_record/railtie.rb:140: warning: method redefined; discarding old extend_message
  ~/projects/rails/activerecord/lib/active_record/errors.rb:104: warning: previous definition of extend_message was here
Cast env to symbol, fixes deprecation warning

Warning:

  DEPRECATION WARNING: Passing a string to ActiveRecord::Base.establish_connection for a configuration lookup is deprecated, please pass a symbol (:development) instead.
Merge pull request #13488 from strzalek/fix-railties-warnings

Fix railties warnings
Fix tests names: tokens.yml => secrets.yml