Store version of ProjectTemplate in global.dcf

- [x] Default: Current version
- [x] ~~If missing: 0.5~~ (will be implemented as part of #89)
- [x] Warn if versions are different
- [x] Stub for `migrate.project()` function
- [x] Test
- [x] ~~Auto-update `global.dcf` when bumping package version~~ (now in #91)