Add app-wide alert system

[segeeslice]

Overview

This PR creates a global alert for use across the application. Instead of defining new logic in each dialog to show individual errors, I can now report alerts anywhere via a simple importable addAlert(...) method.

This can be used for positive feedback, errors, warnings, information, etc.

Stories

• As a user, I want to see alerts/errors as they occur without needing to open a console
• As a developer, I want an easily accessible and central way of displaying errors in the UI

Screenshots

Show Screenshots

### Variable modes: Snackbar or Dialog   ### Three other "Severities"   

[segeeslice]

@RobBoeckermann @jsteuver Feel free to check out the UI screenshots and lmk what you think! This will be app-wide so it's very important that the UI and code are clean.

[jsteuver]

Looks solid to me. Is there anything special that needs to be added to the Django server responses in order to trigger the alerts?

For example, on the /user/login endpoint, we are currently returning a 401 for an invalid login. Will this alert system be able to handle this response as is? Or is there any additional information that should be added to the response?

[segeeslice]

@jsteuver nope no backend dependencies, it's completely agnostic about the data it gets.

[segeeslice]

... For example, on the /user/login endpoint, we are currently returning a 401 for an invalid login. Will this alert system be able to handle this response as is? Or is there any additional information that should be added to the response?

@jsteuver speaking of this, do we have documentation on what codes are returned when? It would be nice for the front-end to expect any of them and know exactly what it's supposed to mean

[jsteuver]

Not currently, but I can update the API docs with those.

[segeeslice]

I'll make an issue for it