add nighly version notice to welcome page

[shun2wang]

Reopened #5797 which was closed by accidently force push...

Fix: https://github.com/jasp-stats/INTERNAL-jasp/issues/2739

We using a branch name to detecting if it is a nightly build. before merge this we should fix Git detached HEAD of this repo.

Now in the welcom page for both developers and users convenient to know whether they are working with a nightly under testing. because we did see in the issue that some users "accidentally" used the test version in the production environment.

[JorisGoosen]

The nightlies page does start with a warning:

Maybe I should just make the "Use at your own risk" way bigger?

There is also the problem of HEAD indeed, the settings of buildbot look sensible to me and @RensDofferhoff but we have no idea why it keeps doing that.

Also there is still the problem where sometimes we did a special release for only flatpak from a different branch than stable/development which would not work well with this PR.

I do agree it would be awesome if the nightly would look different from the release.

I guess if the release always follows the pattern of:

• development changes go to stable
• we build the release from stable

@boutinb @RensDofferhoff