Update UsersController#activity action to return json/html response based on feature flag

Currently app/controllers/users_controller.rb has 8 actions that are used for the user tabs. These actions are activity, groups, contributed, projects, starred, snippets, followers, and following. Currently these actions return HTML that is generated from a HAML partial. We need them to return JSON so that our new Vue components can build the HTML.

This issue is responsible for making changes to the activity action.

Implementation plan

1. In render.json block of UsersController#activity add a check for the profile_tabs_vue feature flag introduced in #378688 (closed)
2. If the feature flag is disabled return what is currently implemented
3. If the feature flag is enabled return the collection as JSON

Availability and Testing

• Since this will be introduced behind an already existing feature flag that is not yet enabled by default, please also create a draft MR that includes these changes with the feature flag definition file set to default_enabled: true. This will then trigger the ee:instance-parallel and ee:instance-parallel-ff-inverse jobs within e2e:package-and-test to E2E test the changes with the FF both on and off (see https://docs.gitlab.com/ee/development/testing_guide/end_to_end/feature_flags.html#automatic-test-execution-when-a-feature-flag-definition-changes for more details)