UX simplification of initiatives for broader use cases

Here is a list of small improvements that we are planning to make on Decidim's master : - Enable global scope selection and signature threshold - Admin option to disable promoting committee - Simplify initiative creation wizard when there is only one option available (1 type of initiative, 1 type of signature collection, only global scope signature threshold) Related Github issue here : https://github.com/OpenSourcePolitics/dev/issues/5