Improve Live Data rendering time when the less cache is not populated

When the cache is empty (e.g., after a server restart or a skin update), compiling a less file to css takes a lot time.

This show up especially when visiting pages with a Live Data macro, where displaying the Live Data can take up to 20 seconds.

This has two negative consequences:

1. the first user to request a less file has a bad user experience
2. this increase the changes of having functional tests to fail by timeout because the UI (in particular the Live Data) takes too much time to be displayed, creating a lot of flickers.

The possible solutions are:

• improve the less compilation time (XWIKI-18310)
• compile the less resources ahead of time
• decrease the number of less resources to compile, since their is a static cost that does not depend on the size of the less file to compile (since we include skin.less implicitly)