Display new Enums in variable pages

[Anna-Livia]

Connected to openfisca/openfisca-france#831

A enum variable page now displays a keyword/description table to indicate to users which keyword they should choose.

[sandcha]

Environment updated ; Chrome 62.0.3202.94 needed a chromedriver > 2.30 (aka 2.33).

One integration test error that seems quick to fix: Found text "celibataire" and no value in VariableComponent.firstPossibleValue, which does not match "/marie/".

Besides, is the following line still true It does not have a formula and therefore is not computable. It can only be used as an input for a computation.? @fpagnoux @guillett Could an enum value be deduced during computation?

[guillett]

Yes, zone_apl is such an Enum.

[bonjourmauko]

Rebase + fixup