Fixes incident type plugin metadata UI

[kevgliss]

No description provided.

[mvilanova]

Pulled the PR locally. Getting none is not an allowed value when trying to save. Also, the + button to add an item does not work.

[mvilanova]

Looks like the PluginMetadata Pydantic model expects slug but we're not providing it:

{"detail":[{"loc":["body","plugin_metadata",0,"slug"],"msg":"none is not an allowed value","type":"type_error.none.not_allowed"}]}