Add concretise and containerise aliases for our UK users

From a user:

> Aargh.
> ```
> ==> Error: concretise is not a recognized Spack command or extension command; check with `spack commands`.
> ```

To make things easier for our friends in the UK, this adds `concretise` and
`containerise` aliases for the `spack concretize` and `spack containerize` commands.

- [x] add aliases
- [x] update completions

lib/spack/spack/main.py

@@ -51,7 +51,7 @@
 stat_names = pstats.Stats.sort_arg_dict_default
 
 #: top-level aliases for Spack commands
-aliases = {"rm": "remove"}
+aliases = {"concretise": "concretize", "containerise": "containerize", "rm": "remove"}
 
 #: help levels in order of detail (i.e., number of commands shown)
 levels = ["short", "long"]