Expand instance-level templates to cover .gitlab-ci.yml, .gitignore and Dockerfile

What does this MR do?

In %11.3, we introduced a custom instance-level templates repository for license templates: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6631

This MR expands support for custom instance-level templates to three other types of file.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

What are the relevant issue numbers?

Closes #5306 (closed)

app/finders/template_finder.rb

+class TemplateFinder
+  VENDORED_TEMPLATES = {
+    dockerfiles: ::Gitlab::Template::DockerfileTemplate,
+    gitignores: ::Gitlab::Template::GitignoreTemplate,
+    gitlab_ci_ymls: ::Gitlab::Template::GitlabCiYmlTemplate
+  }.freeze
+
+  attr_reader :type, :params
+
+  attr_reader :vendored_templates
+  private :vendored_templates
+
+  def initialize(type, params = {})
+    @type = type
+    @params = params
+
+    @vendored_templates = VENDORED_TEMPLATES.fetch(type)
+  end
+
+  def execute
+    if params[:name]
+      vendored_templates.find(params[:name])
+    else
+      vendored_templates.all
+    end
+  end
+end