Ability to specify multiple contexts when defining a validation.

Example:

validates_presence_of :name, on: [:update, :custom_validation_context]
Rails config for raise on missing translations

Add a config to setup whether raise exception for missing translation or
not.
Merge pull request #13754 from vpuzzella/master

Ability to specify multiple contexts when defining a validation.
mention #13314 in 4.1 release notes. refs #12140. [ci skip]
Merge tests about multiple validation contexts
Use the new clear_validators! api to reset validators in tests
Fix doc markup of clear_validators!
docs, remove deprecation entry in favor of abe6484. [ci skip]
Clear filtered request attributes between requests in tests

The request attributes filtered_parameters, filtered_env and filtered_path
are memoized for performance reasons. However this can cause unusual
behavior in tests where there are multiple calls to get, post, etc.

Fixes #13803.
Merge pull request #13832 from kassio/setup-for-raise-missing-translations

Rails config for raise on missing translations
use the new clear_validators! api everywhere to reset validators in tests
Merge pull request #13855 from kuldeepaggarwal/reset_validators

use the new clear_validators! api everywhere to reset validators in tests