Replace LeftAndMain extensions with raw config wherever possible.

[GuySartorelli]

Some of our modules use an Extension class to add i18n js, normal js, and css to LeftAndMain. E.g. this extension in asset-admin.

In silverstripe/silverstripe-admin#1678 we introduced a new LeftAndMain.extra_requirements_i18n configuration property.

This means we can now add all javascript, css, and i18n js to LeftAndMain via configuration. We should not have entire extensions dedicated to that, it's too hamfisted.

Acceptance criteria

• Identify instances where there are LeftAndMain extensions that could be replaced with yml
• Replace the classes with yml

CMS 5 PRs

• API Deprecate extension to be replaced with config silverstripe-sharedraftcontent#255
• API Deprecate extension to be replaced with config silverstripe-asset-admin#1489
• API Deprecate extension to be replaced with config silverstripe-campaign-admin#325
• API Deprecate extension to be replaced with config silverstripe-elemental#1243

CMS 6 PRs

Important

Don't merge these until the CMS 5 PRs have been merged up and the PRs have been rebased.

• API Replace extension with config silverstripe-sharedraftcontent#254
• API Replace extension with config silverstripe-asset-admin#1488
• API Replace extension with config silverstripe-campaign-admin#324
• API Replace extension with config silverstripe-elemental#1242