Theme: Improve rendered artifact filesize and loading time

[marcuspollio]

While the current theme code is fairly decent (average of 400kB per page on 5K pages and almost 1K processed images, total of 200MB raw non-brotlied), there are many improvements and optimizations possible, specially important as the website may be viewed many thousand times per day around the world:

Template:

• Optimize images and assets filesize, at the expense of build time
• Reduce total page count, taxonomy terms and pagers.
• Improve resources minification, loading and synchronization: images, JS, CSS, etc.
• Improve HTML and CSS structure with performance in mind

Config:

• Optimize settings and config