Provide an empty less/custom.less that can be overridden easily from XWikiSkins objects

If you want to build a custom skin on top of Flamingo you need to:
1. override 'less/style.less.vm'
2. add a new import at the end of the list, like '@import "custom"'
3. write your own 'less/custom.less'

The idea is to add the import to 'custom' (or another name) to the 'style.less.vm' by default. This way the users that want to reuse Flamingo just need to make step 3.

It's a very similar idea to what we do with 'layoutExtraVars.vm' or with the 'advanced' section of Flamingo Themes.

The only rules for the custom.less is that it needs to be the last in the list of imports and it needs to be empty by default.