Draft: Merge breaking map changes

To be merged right before the Xonotic 0.9 release.
Do not squash commits.

Changes that were merged to pending-release already:

• Small collision geometry changes in Fuse and Space-Elevator.
• !174 (merged): Major Opium refactor, based on an unreleased version that cityy found on an old hard drive.
• !187 (merged): Fix weapon projectiles passing through certain crates on Solarium, make crates collisions consistent.

Changes targeting pending-release but not merged yet:

• !177: Rename weapon filenames in entities.ent to avoid confusions.