Support Ajax endpoints for FeatureFlagsController

What does this MR do?

This MR supports the following Ajax endpoints for FeatureFlagsController

• show
• create
• update
• destroy

Name	Endpoints	params
show	GET "-/feature_flags/:id"	N/A
create	POST "-/feature_flags"	{ "operations_feature_flags": { "name": "String", "active": "Boolean" } }
update	PUT "-/feature_flags/:id"	{ "operations_feature_flags": { "name": "String", "active": "Boolean" } }
destroy	DELETE "-/feature_flags/:id"	N/A

Those endpoints are necessary for Vue.js refactoring on feature flag pages.

Those endpoints are not necessary for the implementation on https://gitlab.com/gitlab-org/gitlab-ee/issues/8621.

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/8621

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• 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?
• Security reports checked/validated by reviewer