WebP support

Changes for Theme Developers

• Webp is now supported as a conversion format ( {{ post.thumbnail.src | towebp }} ) @mhz-tamb @pascalknecht #1638 #1777 #1780
• Timber now recognizes that SVGs shouldn't be resized as if they are rasters (for retina, etc.) @palmiak #1726 #1736

Fixes and improvements

• Clean-up on i18n function calls @drzraf #1753
• Fixed some odd port handling @pascalknecht #1760
• Fixed how terms are retrived through a post @shvlv #1729