refactor ScopePage, add test

Created by: sqs

The ScopePage shows a search bar for a specific search scope defined in settings. It is a way to make a nice landing page for a specific kind of search.

• Convert to React.FunctionComponent
• Add test
• Remove custom CSS, use Bootstrap

Example

Given the following settings:

{
  "search.scopes": [
    {
      "id": "sourcegraph-oss",
      "name": "Sourcegraph open-source code search",
      "description": "Search across all Sourcegraph open-source repositories.",
      "value": "repo:^github\\.com/"
      }
  ]
}