Resolve "Remind author do something only when he has permission"

What does this MR do and why?

If a member does not have permission to add label to an issue or MR in gitlab-cn, we should not remind him to add label. So we should modify the logic of "applicable?" functions in relative processors.

Screenshots or screen recordings

When resource author is not a gitlab-cn group member, jihubot won't remind him of adding tags:

otherwise a reminding comment will appear:

How to set up and validate locally

Use GDK to validate this new feature locally.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Add function to get all members who have permission to add label to issue or MR in gitlab-cn group.
• Use a cache to store these members' info.
• Modify applicable? function in relative processors.
• Add test case for new feature.
• Use RSpec to test new feature and fix bugs.

Closes #21 (closed)