Make Parameter and Scale description not required anymore

[cbenz]

Precedes #59

In order to keep PRs atomic, I create this one dedicated to fix a warning I didn't see during development:

[0] Warning: Failed prop type: Invalid prop `parameters[749]` supplied to `HomePage`.
[0]     in HomePage (created by RouterContext)
[0]     in RouterContext
[0]     in IntlProvider
[0] Warning: Failed prop type: Invalid prop `parameters[749]` supplied to `App`.
[0]     in App (created by RouterContext)
[0]     in RouterContext
[0]     in IntlProvider

The prop-types system is specific to React and overlaps with a type-checking system; but it's here so better fix it.