Remove unneeded content types being added to Campaigns

[newleeland]

Description

When a page with content blocks is added to a campaign, extra content types appear within the campaign, which are really confusing for the user. i.e. Sections called "Elemental Areas" and "File links" appear along with "pages and files"

Adding:

• Pages (with content blocks) should be placed under "Pages" section within the campaigns
• standalone Content blocks should be placed under "Content blocks" section [future: content blocks can't be added to campaigns as a standalone atm]

Steps to Reproduce

1. In Campaigns, click Add Campaign
2. Go to a page with content blocks.
3. Click Add to Campaign that you just created
4. Go the Campaign and view the extra items

Affected Version

SilverStripe 4.x

[newleeland]

moved to https://github.com/silverstripe/silverstripe-campaign-admin/issues/128