ci: add gitlab config for automatic denali build

[wpbonelli]

• add gitlab-ci.yml with denali deploy job, triggered on push to default branch on internal USGS GitLab mirror
• add update-module.slurm.batch job to update denali mf6 module
• move hardcoded info to env vars set via git forge (github or gitlab) variables

[jdhughes-usgs]

Would it be better to put .gitlab-ci.yml in a .gitlab folder? Don't know if this would work.

[wpbonelli]

I'd prefer that too but the docs say it needs to be in the root

[jdhughes-usgs]

I'd prefer that too but the docs say it needs to be in the root

Ok. Looks good to merge then.