Improve Lighthouse score for article page

[arvindpdmn]

• Don't load MathJAX if article doesn't use it (defeats concat JS); MathJAX from CDNJS blocks main thread
• Pre-load fonts
• Micropat (addtoany) blocking main thread
• Improve caching of JS files
• Improve __devopedia.js execution time
• Upgrade to latest jQuery (security reasons)
• Let Milestones come below Discussion on mobile view
• Try removing milestones styling and animations
• Server side: try moving PHP handler from CGI to FPM
• Is there perf drop due to acronyms?

[arvindpdmn]

Inconsistent performance numbers from Lighthouse. Made some changes but no big change in numbers at webpagetest.org. More testing needed.

Ignored for now:

• Didn't notice improvement due to removing acronyms.
• Explored loading minimal font-awesome fonts by customizing via https://icomoon.io/. Customization is also cumbersome. There's always a chance of leaving out some font. Score probably won't change much. Ignoring for now.
• Replace old version of jQuery 1.12.4 coming from media/jui/js: can use jQuery: tried v2.2.4 and seen to have same vulnerabilities as pointed by Lighthouse: can't move to latest due to dependencies with com_widgetkit and these errors.
• Resize cc-by-sa-logo.png: only 10KB and reused in About page at the correct resolution
• Optimize jQuery UI by including only Sortable support as used in Ref Formatting Tool during article edit: https://jqueryui.com/download/ : link not working; anyway, affects only article edit page, so ignore
• Try to remove bootstrap CSS in admin panel for the template. Instead, remove bootstrap.min.js in Gulp. Error seen in console log. Remove bootstrap.css: affects a few elements across pages. Keeping bootstrap.
• Preload fonts from another domain. See https://developers.google.com/fonts/docs/getting_started

Completed:

• Removed animations in milestones and article stats though measurements didn't show improvement.
• Use preconnect for https://stats.g.doubleclick.net, https://www.google-analytics.com and others.
• MathJax: load only if article has equations
• Reduce DOM elements by removing duplication of Milestones. Do this during redesign: Redesign article page for better usability and performance #490
• Caching of article pages and related changes

[arvindpdmn]

Done