[FIX] Do not top up all @imports before less/sass compilation

This was preventing the bundles fragments to be restored after
compilation
[IMP] Use bundle compilation output instead of fragment recomposition
[IMP] recompose bundle only if needed
[IMP] Css preprocessors can now be mixed up in the same bundle
Merge pull request #1765 from odoo-dev/master-fix-less-compilation-errors-fme

[FIX] Asset bundles compilation bug + support for mixed preprocessing languages

- moving imports to the top of the source to compile breaks the assets recomposition trough markers
- improved performance by avoiding useless assets fragments recomposition
- added support for mixed preprocessing languages in the same bundle
[IMP] code auto reload: logger.error in case of syntax error
[FIX] Do not alter @import statements of saas/less
[FIX] Fix website theme slowness
[FIX] --dev mode crashes on read_template ormcache invalidation
[IMP] bundles: display warning for forbidden local @imports
[IMP] hr: first employee is named after admin
[IMP] bundles: also display css errors on debug mode
[ADD] website_forum: Adding Social Network Sharing functionality to share Questions and Answers