RDF importer should suggest/require ending namespace with slash or hash

[zerocrates]

Both the trailing slash and hash on a namespace URL are easy to omit and will usually still result in the right number of vocab members, but can cause issues down the line with things that do care about the actual namespace URL being correct.

[jimsafley]

Another approach is to allow administrators to edit the namespace URI when editing a vocabulary. That way they can correct the trailing character, update the schema (e.g. http:// to https://), and make other minor corrections themselves. Currently the URI is read-only, which requires users to edit it directly in the database should they encounter irregularities.

We were probably too cautious in our initial decision to exclude the URI from the form. It would open the door to invalid namespaces, but a warning that explains the consequences of changing it should suffice.

[jimsafley]

This fix is part of #2093