CE uses application_settings.help_page_text but EE uses application_settings.help_text

Proposal

1. Add help_page_text to the API (in CE and EE).
2. Clarify what help_page_text is used for (in CE and EE).
3. Remove help_text from EE.

Original description

As seen in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2137

app/views/help/index.html.haml outputs application_settings.help_text in EE, and application_settings.help_page_text in CE.

The former column doesn't exist in the CE schema at all. It seems both CE and EE write to help_page_text.

Let's remove it from EE and use help_page_text throughout.

The only question is how best to ensure we don't wipe out help page text that has been placed into help_text by accident. How long ago did EE start writing to help_page_text?

/cc @rymai @axil