GET Description Templates via API

Problem to solve

Description templates are stored in the repo. It's possible to retrieve them via the files API, but it could be nice to see the description templates indexed and retrievable via their own API

Proposal

• Index available description templates for /issues and /merge_requests
• All retrieval of individual description templates for /issues and /merge_requests via the filename/ template name

Links / references

https://gitlab.com/gitlab-org/gitlab-triage/issues/79

Files API:

• https://docs.gitlab.com/ce/api/repository_files.html#get-file-from-repository

Description templates docs:

• https://docs.gitlab.com/ee/user/project/description_templates.html#doc-nav