Dont allow creation of elements with a name that already exists in the manifest

[dlett]

No description provided.

[bdeanindy]

While doing this, also ensure that sub-app names are in lowercase during creation weebly add <type> and the corresponding entries in manifest.json.

Failures occur when there isn't consistency between the sub-app name references from the various related methods in the CLI such as: weebly list, weebly delete <type> <name>.