Allow command correlation to a saga and allow multiple endpoints to send the same command

Correlating commands to a Saga - Implementation for https://github.com/particular/requirements/issues/75
Fixes - https://github.com/Particular/ServiceMatrix/issues/392
Features implemented:

- Gets rid of the concept of a command having the single sender and handler components with specific names. Commands can have multiple sender components, and handler components can handle multiple commands (and events).
- When sending a command from an endpoint, the user can choose whether the handler for a new command should be a new component or an existing one. In the later case, the user is prompted about making the component a saga.
- New feature to send a command from an endpoint. In this case the user picks the endpoints from which to send the command. Endpoints hosting components which send the command are shown as selected but are disabled.
- Removing a command prompts the user whether to remove components associated to the command. If a related component is only related to the deleted command, it's selected by default for deletion but users can override the defaults.
- Add subtle hint that users can pick or write new content for editable combo boxes.