Fixing showing activate link after installing Add On

[dparker1005]

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Changes proposed in this Pull Request:

Alternate to #3254

After installing an Add On via the Memberships > Add Ons page, we try to find an "activate" link to show the user so that they can immediately activate the plugin. This functionality was not working. Upon testing, I found that this may have been due to the Code Snippets having a button with the .button-primary class that confused our code in thinking that was the "activate" button.

This PR fixes the issue by specifically looking for a link with plugins.php?action=activate&plugin= to greatly increase our chances of finding the correct activation link.

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Changelog entry

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