Restrict feed name allowed characters

Feed names end up as part of a crossbar URI for pub/sub. crossbar has restrictions on what characters are allowed in these URIs as defined here: https://crossbar.io/docs/URI-Format/

We should restrict feed names to these rules so that users can't accidentally generate improper URI names.

These are the rules from crossbar:

• Allowed characters are lower case letters, numbers, ‘.’ and ‘_’. (Lower case letters are used to ensure compatibility with languages which have case-insensitive identifiers.)
• There can be no non-empty parts between the separating full-stops, e.g. com.expressweasel.user..start is not a valid URI. (It could be used as part of patter-based subscriptions/registrations though.)
• wamp as an inital URI part is not allowed, since this is reserved for URIs predefined with the WAMP protocol itself.
• crossbar as an initial URI part is not allowed, since this is used by Crossbar.io itself for internal messaging.

No file or test is identified in the issue. Start by tracing where feed names are accepted and converted into Crossbar pub/sub URIs, then compare that path with the linked Crossbar URI rules; done means invalid names are rejected and valid names remain accepted.