Make notification message input multi-line

[davelopez]

This is a very minor change, but the single-line text input was messing with the markdown format if you updated an existing notification message from the Notifications Admin Panel.

Before

After

How to test the changes?

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[martenson]

thanks @davelopez