Partially reverted bits from r108300. Not allowing underscores is too restrictive...

Partially reverted bits from r108300. Not allowing underscores is too restrictive and breaks on default DB name value suggested by installer. Azure can use resolveContainerName() to enforce its extra restrictions. A new resolveWikiId() function was added to allow for it to mangle the wiki ID by translating underscores. Also restored previous size limit in isValidContainerName() since it makes sense to either go all the way lowest-common-denominator or not here (in this case not). Preserved "first char is a alphanumeric" check since it's pretty weird to have anything else.