Add a confirmation that the To-do was added successfully

[amyblais]

Today I assigned a to-do for another person, but I never got a confirmation message saying that the to-do had been added successfully. I ended up adding the to-do 3 times because I didn't know that the first time was succesful. I asked the other person and they said that they had gotten notifications for all 3 attempts.

Repro steps:

1. Open the To-do RHS.
2. Click on + Add To-do.
3. Assign the To-do to another person and add a title.
4. Click Save.
5. The modal closes.

Observed:
You have no idea if the to-do was added successfully for the other person.

[mickmister]

@asaadmahmood Is this possibly a regression from recent changes? cc @DHaussermann