Webflasher only with github login

[eh2k]

In case you are wondering why you now have to log in for the Webflasher.

I would like to use the infrastructure of github (github API) more in the future.

The Github API is quite handy to list the available versions, or to access the compiled engine files, that stored in the repo.
My goal is also to use the github actions to compile the FV1 effects or similar (work in progress).

I think that users without a Github account might also have a reason to sign up. And they would be more likely to report any issues or give feedback in general.