Use GitHub user's ID instead of username to store contributors activity

[rithviknishad]

Problem

When a contributor changes the GitHub username, their activities are recorded as a new profile under their new username instead.

For example, see:

• https://contributors.ohc.network/contributors/cp-Coder
• https://contributors.ohc.network/contributors/aeswibon

Proposed solution

We could use the github user's id instead of username to store the scrapped data. However, the slug for profile route could remain username itself and this could be resolved to the id internally.