Migrate accessibility rubocop rule ImageHasAlt from dotcom to erblint-github

[bolonio]

Context

The motivation of rubocop-github is to open-source our accessibility rubocop rules so non-GitHub people can benefit from them, have a space to provide comprehensive rule documentation, and also allow rules to be shared between Rails projects.

This PR migrates the accessibility rule ImageHasAlt from dotcom to rubocop-github

Related Issue

• https://github.com/github/accessibility/issues/1430

[khiga8]

We should also update one of the config files so these accessibility rules are pulled in easily. Maybe config/default.yml?

Or do you think it should be in an accessibility-specific one? Maybe a new config/accessibility.yml?

[khiga8]

Could we create a new accessibility folder to house the accessibility rules?

This will ensure the file path reflects the module structure.

module RuboCop
  module Cop
    module GitHub
      module Accessibility

Then the file names shouldn't include rails_

[owenniblock]

Looks good to me!

[khiga8]

Awesomeeee