Fix snippets API not working with visibility level

Closes #66050

See merge request gitlab-org/gitlab-ce!32286

Standardize use of `content` parameter in snippets API

Closes #66673

See merge request gitlab-org/gitlab-ce!32296

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

When a restricted visibility level of `private` is set in the instance,
creating a snippet with the `visibility` level would always fail.
This happened because:

1. `params[:visibility]` was a string (e.g. "public")
2. `CreateSnippetService` and `UpdateSnippetService` only looked
   at `params[:visibility_level]`, which was `nil`.

To fix this, we:

1. Make `CreateSnippetService` look at the newly-built
   `snippet.visibility_level`, since the right value is assigned by the
   `VisibilityLevel#visibility=` method.
2. Modify `UpdateSnippetService` to handle both `visibility_level` and
`visibility` parameters.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66050

Instead of sending varied data to Gitaly, and making Gitaly construct
various messages, build the messages first and have Gitaly print
either basic messages or alert messages, in the order they come.

Depends on https://gitlab.com/gitlab-org/gitaly/merge_requests/1410

Promote Label to Group Label via API endpoint

Closes #57657

See merge request gitlab-org/gitlab-ce!25218

Fix N+1 Gitaly calls in /api/v4/projects/:id/issues

See merge request gitlab-org/gitlab-ce!32171

The `GraphQL::Function` has been deprecated in
favor of resolvers.