Improve Embeds, Major Refactor/Cleanup

[solonovamax]

Improves embeds and a major refactor/cleanup of basically everything.

This improves embeds based off of the suggestions provided via #27

Here's a list of relevant issues

• closes Suggestions: Improve display of webhook #27
• How to get notifications to match traditional notifications when moving to declarative pipelines? #131
• fixes Feature Request: Parity with pipeline implementation #13

Here are the changes to the embed:

• Move everything to embed fields
• Shortened the artifacts list by using inline links
• Some other misc cleanup

For the refactors/cleanup:

• Remove code for discord webhook, instead use discord-webhooks
• Move all the code for building embeds to a shared util class

Testing done

Honestly I have no clue how to do testing with Jenkins plugins. Would love feedback for that if tests are needed, however since there were already basically no tests I felt it wasn't super needed.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue