Make "big-merge" the default

[x] Disable standalone builds
[x] Continue building big-merge into llvm-snapshots-big-merge-YYYYMMDD on copr but fork them into llvm-snapshots
[x] Update copr project documentation about the new name
[x] Check if we need to adapt build time diagrams
[x] Move code from https://src.fedoraproject.org/fork/kkleine/rpms/llvm/tree/big-merge to https://src.fedoraproject.org/rpms/llvm/tree/big-merge (I moved my commits around and squashed them. You MUST rebase your patches)
[x] Get rid of llvm-snapshot-builder
[ ] Get documentation in README.adoc in good shape
[x] Adjust branch protection rules that no packit builds for llvm-snapshot-builder are required
[x] Decide what to do with lld. It was packaged in standalone mode but not in big-merge. Options are to continue building it standalone or merge it.

fedora-llvm-team / llvm-snapshots