WIP: Add check for `skip_prompt=yes` environmental var

What does this MR do?

Normally, running gitlab-rake import:github the user is prompted for input before the import if performed.

This MR allows for the setting of an environmental variable, skip_prompt with a value of yes which will cause the input prompt to be skipped, continuing with the import.

There are no additional security implications or risks as the affected functionality is limited to an environment variable and user prompt.

The functionality of this environmental variable check allows for streamlined importing of GitHub repos into GitLab in automated deployments to fully populate a GitLab instance. Currently, a live TTY device is required for the user input prompt so the use of echo, expect, or yes do not meet the needs in a scripted setting.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

No risks introduced by skipping user input prompt on a GitHub repo import. No cross-browser testing required as the interface is drawn from the gitlab-rake shell command.

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers