[RFR] Custom menu

[fzaninotto]

• Add a Menu model
• Add a compatibility layer between Entity.menuView() and Menu
• Change all inner links to put the entity name first (e.f. '#/list/posts/' becomes '#/posts/list') (eases url recognition)
• Optimize perf
• write doc

[fzaninotto]

Switching to RFR

[alexisjanvier]

@fzaninotto I'm certainly not the most qualified reviewer on es6 part, but for me everything is good. Especially as it is tested ... About that, all tests pass on local, but failed on travis. Is this a known issue ?

[fzaninotto]

Yes it is a known problem. e2e tests fail for no apparent reason, and it's a pain to debug. That's also why I didn't add more e2e tests for this PR.

If you're satisfied with the code, I'll let you merge, and I'll push an update to the built files to master right after.