Index entities

[sandcha]

Fixes #80

Adds specific page for each entity Use same url for entity exploration (e.g. http://localhost:2030/person) List entities in home page Adapt search function to look for entities Add link to entity in parameter and variable page

[fpagnoux]

I also have a warning on the variable pages:

checkPropTypes.js:19 Warning: Failed prop type: The prop `entities` is marked as required in `Variable`, but its value is `undefined`.
    in Variable (created by InjectIntl(Variable))
    in InjectIntl(Variable) (created by ParameterOrVariableOrEntityPage)
    in div (created by ParameterOrVariableOrEntityPage)
    in ParameterOrVariableOrEntityPage (created by RouterContext)
    in div (created by App)
    in DocumentTitle (created by SideEffect(DocumentTitle))
    in SideEffect(DocumentTitle) (created by App)
    in App (created by InjectIntl(App))
    in InjectIntl(App) (created by RouterContext)
    in RouterContext (created by Router)
    in Router
    in IntlProvider

[fpagnoux]

Added links from variable pages to entities page: