Ci/improve previews and preview ux

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Description

This PR adds the following improvements:

• On pull request page, keeps a single comment with PR preview link after most recent commit(s) pushed
• Fixes issues with non-working buttons on the homepage if baseURL has a subdir (pr)
• On preview site, adds a link back to the original pull request

Issues with the article shortcode

Our PR previews have issues showing the featured articles. The problem lies in the fact that our PR preview's baseURL has a subdir pr<pr-number>. While creating links to articles, the article shortcode discards the baseURL's subdir at some point.

As part of this PR, I tried to fix this issue, but it is way over my level of expertise.

Related links:

• How to handle URLs when in a subfolder?
• About problem with canonifyurls
• baseURL with subfolder: why is it not working
• BaseURL path (subdir) sometimes ignored by relURL, absURL
• baseURL = mysite.com/test causes wrong menu links like mysite.com/test/test/about

Related

• Is there an easy way to preview the new article? #43

Testing

Tested all changes in this testing environment.

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All changes appear to be working as expected. Merging this PR.