Move webapp.gitlab-ci.yml to ci-templates.

We've copy-pasted .gitlab/webapp.gitlab-ci.yml around a few places now and so we should move it to ci-templates.

Tasks:

• Open MR in ci-templates adding template. The template should pin the poetry and tox versions for stability. (We can update the template over time to update them.)
• Open MR in this repo changing include in .gitlab-ci.yml to use the template.
• Open MRs in the following repos which use the template:
  • https://gitlab.developers.cam.ac.uk/uis/devops/regent-house-ballots/
  • https://gitlab.developers.cam.ac.uk/uis/devops/laboratory-allocator/lab-allocator
  • https://gitlab.developers.cam.ac.uk/uis/devops/iam/identity-photos-service/photo-api
  • https://gitlab.developers.cam.ac.uk/uis/devops/iam/authentication/raven/ucamwebauth-2-saml2
  • https://gitlab.developers.cam.ac.uk/uis/devops/iam/idhub