Add content blocks admin dashboard

[abelemlih]

This PR addresses the following requirement from #1275:

• Add admin ability to edit front page banner

This PR includes the following changes:

• Add gem administrate to use customizable admin dashboards
• Add a dashboard to display and edit content blocks under route /admin/content_blocks
• Only allow logged in admins to read and update content blocks, no user is allowed to delete a content block
• Use value of ContentBlock.homepage_banner to populate the homepage banner, and remove deprecated view _beta_banner.html.erb

Please refer to the following screenshots: