Remove hardcoded Zapier webhook URLs

[jgalea]

Summary

Two Zapier webhook URLs were hardcoded into the plugin source:

• includes/admin-plugins.php:29 — POSTed plugin-deactivation reason + installed-addon list from a Vue feedback modal on wp-admin/plugins.php
• includes/functions.php:409 (wprss_sub_to_newsletter) — POSTed name + email from the post-activation newsletter signup form

Both upstream Zaps have been deactivated, so the URLs are dead in production. They remain visible in shipped source though, which is what this PR cleans up.

What's removed

• includes/admin-plugins.php and its Vue source (js/src/modules/plugins/, css/src/plugins/). The feedback modal was the only consumer of the URL and the only purpose of the file. With the Zap gone, the modal was a no-op that submitted to a dead endpoint. Removing it makes Deactivate on the plugins page work the way core WordPress intends — direct deactivation, no modal, no follow-up prompt.
• The plugins entry from webpack/webpack.vue.js so plugins.min.js / plugins.min.css are no longer built.
• The require_once(WPRSS_INC . 'admin-plugins.php') line from wp-rss-aggregator.php.

What stays

wprss_sub_to_newsletter($name, $email) is kept. The hardcoded Zapier URL is replaced with a filter (wprss_newsletter_subscribe_url) that defaults to an empty string. When empty, the function returns false without making a request and logs an info message. Callers (admin-intro-page.php) ignore the return value and still show the success notice, so end-user UX is the same.

This way the function still has somewhere to land if a new endpoint is wired in later, but the dead URL doesn't ship.

Testing

• php -l passes on includes/functions.php and wp-rss-aggregator.php.
• grep -rn "hooks.zapier" --include="*.php" --include="*.js" --include="*.scss" . returns no matches.
• grep -rn "admin-plugins\|wpra-plugins\|plugins.min" --include="*.php" --include="*.js" . returns no matches.

Notes

• Diff is +13 / -506 (the deletions are mostly the SCSS and Vue files that supported the dead modal).
• No data-collection behaviour change for users on a current install: both endpoints were already silently failing.

[jgalea]

This is the source-level fix for the same issue tracked in the v5 monorepo at RebelCode/wpra#649 (build-pipeline patch tracker at RebelCode/wpra#677). Once this lands on develop, the Makefile patches in wpra#677 become unnecessary.