Share bonus points and reward credit notifications/thank yous to Discord

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What does this MR do and why?

This MR adds DiscordNotificationService to send notifications to Discord for contributor-related events. Follows the "webhook" technique, and took inspiration from https://gitlab.com/gitlab-org/developer-relations/contributor-success/toolbox/-/blob/main/lib/discord_helper.rb:

• Bonus points notifications: Sends a message to Discord when users earn contribution points on level up

• Reward credits notifications: Sends a message when users receive reward credits.

• RSpec tests: Coverage for service methods.

Screenshots or screen recordings

Before	After

Validation steps

/cc @leetickett-gitlab

Closes #72 (closed)