Refactor Slack image icon with svg icon in communities of practice page

[aadilahmed]

Fixes #5829

What changes did you make?

• Replaced img tag for the "Join Slack Channel" button icon with a liquid image element referencing _includes/svg/icon-slack.svg
• Modified CSS to maintain current appearance of page

Why did you make the changes (we will use this info to test)?

• To make sure that only a single slack icon svg is used in the codebase
• Make it easier to render the svg in various colors throughout the website

Screenshots of Proposed Changes Of The Website (if any, please do not screen shot code changes)

Visuals before changes are applied

Visuals after changes are applied

[github-actions]

Want to review this pull request? Take a look at this documentation for a step by step guide!

From your project repository, check out a new branch and test the changes.

git checkout -b aadilahmed-refactor-communities-of-practice-page-5829 gh-pages
git pull https://github.com/aadilahmed/website.git refactor-communities-of-practice-page-5829

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https://github.com/aadilahmed/website/blob/refactor-communities-of-practice-page-5829/CONTRIBUTING.md  

[roslynwythe]

Excellent job @aadilahmed. The PR has well-formed branches, descriptive text and screenshots. The code is correct and clean and it looks great in the browser. Thank you!

[agutiernc]

Review ETA: EOD of 11/16/23
Availability: After 6pm

[agutiernc]

I confirm the following:

• Correct branch and linked issue
• Correct code to display the slack icon
• Changes render in different view modes
• Screenshots display the changes correctly

Outstanding job @aadilahmed! Thanks for contributing!