There was some confusion over whether `code` or `content` is the right
parameter for snippets. Internally, the database stores
`content`. However:
1. Project snippets use `code`. `code` gets remapped in `content` in
`lib/api/project_snippets.rb`.
2. Personal snippets use `content`.
To unify these APIs, allow an alias of `content` to work for project
snippets.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66673
