Update code owner check for assigning code owner.

[riyasingh1004]

Currently, oppiabot gets the list of code owners from the list of requested reviewers.
Often this works, because GitHub automatically requests a review from the code owners but sometimes, the PR author would explicitly request a review from another contributor who isn't a code owner, and oppiabot will incorrectly assign the contributor as a code owner.

This issue aims at using the CODEOWNERS file from the oppia repository to get the appropriate code owners for the pull request based on the files changed.

The CODEOWNERS file can be found at https://github.com/oppia/oppia/blob/develop/.github/CODEOWNERS.

[vojtechjelinek]

@jameesjohn Can you help here?

[jameesjohn]

Done.

[vojtechjelinek]

@jameesjohn I'm still a bit unsure what this issue exactly means? What is the problem with assigning reviewers as code owners even when they arent code owners?

[seanlip]

I don't understand the problem here. Closing.