fix: allow deleting non existing flag

[Gladear]

Attempt to fix #26

It's pretty hard to test if it actually works since I don't have multiple nodes to test, so sorry if the fix isn't correct 😕

[tompave]

Apologies for the long wait, and thank you for giving this a shot.

I'm not sure if this solution is the right one, though.
Did it work locally? Unfortunaly CI can't run on this PR because it's based on an older commit of the master branch where the CI image was too old. If you could merge the latest master in and push, CI will be able to run.

[tompave]

Hi @Gladear , thank you for iterating on this.
I'd ask to please not commit --amend and force push. It blows away the PR history, and it makes it difficult to see what changed. You can simply add a new incremental commit and push.

[tompave]

Ok, I think this works in theory, thank you.

However, I don't think that the error from #26 comes only from the delete "/flags/:name/boolean" endpoint you've modified. As I mentioned here, the problem is more widespread. I think that all endpoints need to switch to logic that ensures that String.to_existing_atom/1 doens't just rely on what's available in memory.

From the original message:

It's pretty hard to test if it actually works since I don't have multiple nodes to test, so sorry if the fix isn't correct

On the master branch of FWF I've added some instructions on how to run multiple nodes: https://github.com/tompave/fun_with_flags#working-with-pubsub-locally

But I don't think you need that. You can restart the FWF.UI app (iex -S mix) and go directly to a flag details page without first loading the homepage.