Use LFS for project templates

The following discussion from gitlab-ce!25553 should be addressed:

• @nick.thomas started a discussion:

  One does wonder whether we should start using LFS for these project templates. The repository is going to rapidly balloon in size otherwise 🤔

  One for a follow-up issue, methinks.

Currently, project templates are stored as .tar.gz files in vendor/project_templates. They are 4.5MiB and we keep adding new ones. Further, they will require regular updates with new versions. Every time a version bump happens, the old version stays in the repository's history - forever! 😱

I think we should instead use LFS for these files. It's what it's designed to do, and should help us to keep the size of the repository down over time.

Thoughts @grzesiek @rymai ? (picking 2 other maintainers at random)