[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
[IMP] Cache bundles in ir.attachments instead of a LRU

Forward ported from 8.0's commit: 8402625
[FIX] mail: bad test
[IMP] hr: strenghtening of write()
[FIX] bundles: fix compiled css assets recomposition

- Do not include css in less source because malformed css will break
  less compilation
- Always recompose compiled css, even if not in debug mode
- Less and sass compilers mode @at-rules on top in order to comply
  to css 2.1, those @at-rules where left-over during the recompostion
  because they lost their marker. Those are now handled as virtual
  inline stylesheets.
[FIX] ir.attachments of compiled css assets not properly invalidated
Update __openerp__.py

Little mistake in the descreption seen during translation.
[FIX] website.theme: when the user does not raise an exception. Display error message when the user install or customize a theme.
[FIX] tour: don't display all log when log an error