Custom global validators

[fxisco]

Description

This PR adds global validators to kiln. To enable this in the site we can include validators the same way we do for components: add a validator to window.kiln.validators. The main difference relies on the validator object, is needed to include scope: metadata to identify it as a global validator.

e.g.:

Metadata validation for all pages

include scope: 'metadata' in object and on type indicate if is error or warning

'use strict';

module.exports = {
  label: 'Lorem Ipsum',
  description: 'Lorem ipsum dolor sit amet, consectetur...',
  type: 'error',
  scope: 'metadata',
  validate(meta) {
    if (meta.authors.length < 1) {
      return [{
        preview: 'Blah blah'
      }];
    }

    return [];
  }
};

Metadata validation for specific page

include scope: 'metadata' in object. NOTE: include uri: <URI>,

'use strict';

module.exports = {
  label: 'Lorem ipsum',
  description: 'Lorem ipsum dolor sit amet, consectetur...',
  type: 'error',
  scope: 'metadata',
  uri: 'cjp2sl6c50000cyv2gt73xbwt',
  validate(meta) {
    if (meta.authors.length < 1) {
      return [{
        preview: 'Blah blah'
      }];
    }

    return [];
  }
};

Approach taken

• Filter validations using condition scope , type and uri (for specific page).
• Running validators with a different function that fetch the metadata first and then use this info to validate all the metadata validators.

Note: we are able to create metadata warnings if necessary.

Ticket

Closes #1368

WIP

• Missing tests
• Missing comments

[coveralls]

Coverage increased (+0.2%) to 34.49% when pulling 201ae01 on validators into f5fe40e on master.

[fxisco]

@scottnash we should not add anything special. Here an example(https://github.com/nymag/sites/pull/7308/files)

if we don't set scope, it is consider a global validator, i.e. all the ones that already exist. To be able to make validations with the metadata we should specify scope: 'metadata' and what we are going to receive as argument in the validate functions will we the metadata and not the component state as normal.

[scottnash]

Branch is out of date with master now, but other than that it looks good.