#Compose Switching to an Identity with no signature should remove previous signature

[castaway]

Describe the bug
When starting to compose an email, if the default identity has a signature, we insert it. If we then switch to one with no signature, the original signature should be removed

To Reproduce
Steps to reproduce the behavior:

1. Make sure the default identity has a signature
2. Go to Compose
3. Choose another identity with no signature
4. Original signature should be removed