Harmonize configuration collection names and language codes

Problem/Motivation

• While working on #2248767: Use fast, local cache back-end (APCu, if available) for low-write caches (bootstrap, discovery, and config) → , I ran into the problem that LanguageConfigFactoryOverride::createConfigCollectionName() creates a collection name that violates the documented (but not enforced except when a specific backend chokes on a character it can't cope with), restriction on collection names in StorageInterface::createCollection(), because language codes contain hyphens.
• Mapping langcode hyphens to collection name underscores seems natural, except LanguageFormBase::validateCommon() currently allows underscores too, despite that not being allowed by the RFC referenced by http://www.w3.org/International/articles/language-tags/.
• In reading that RFC, I found that it allows numbers in language codes. Would numbers be problematic in config collection names?

Proposed resolution

Figure out how to harmonize all of the above.

Remaining tasks

• See what's in the patch.
• Do we want to add validation in implementors of StorageInterface that the passed in collection name conforms to what the interface docs say it should?
• Do we need additional tests of both langcode validation and collection name validation?

User interface changes

API changes