Add Fluxer account as a login provider on Weblate

The current login option available on https://weblate.fluxer.tools/ is just GitHub. There is no other login method available.

Given the recent push by the Fluxer team to move away from GitHub, I believe it would make sense if you could log into the Weblate account with your own Fluxer account instead of relying on a GitHub account. For context, here's a direct quote from the post linked above:

It also makes us less dependent on centralised American services such as GitHub, which plenty of you would rather not sign up for just to report a bug.

I believe it should not be replaced, as current users who already signed up with their GitHub account can still use it with no problems. This proposal is just asking for a second login option, not replacing the current one.