Have a configuration for compiling our custom version of Bootstrap

From their website, Bootstrap allows compiling your own custom version which could:

• Be lighter by adding only the components that you need (faster downloads).
• Use our custom color scheme and typography straight from the beginning (less code, less work, less debugging).

This might imply learning a bit of JSON and Less but otherwise don’t require a deep knowledge of how our website is build, so I’m marking this as “easy”.

The idea would be to cover with this custom version, the needs of all our pages with Bootstrap. So far that’s /install (and subpages), /upgrade, and /donate.

Parent Task: #15749

Related issues

• Related to #10176 (closed)
• Related to #15800

Original created by @sajolida on 11821 (Redmine)