Fixes deprecation warning:

```
DEPRECATION WARNING: The success? predicate is deprecated and
will be removed in Rails 6.0.
Please use successful? as provided by Rack::Response::Helpers.
```

Splits auto-refreshing of MR widget into 2 requests:

- the one which uses etag-caching and invalidates the fields on change
- the one without caching

The idea is to gradually move all the fields to etag-cached endpoint

The actions tracked in the web IDE are:
- creation of commits
- creation of merge requests
- projects loaded

Prefer `json_response` where applicable.

This commits extracts /merge_requests/1.json?serializer=widget
Into a separate /merge_requests/1/widget.json endpoint
This will allow to use caching for this request

Adds frozen string to the following:

* spec/bin/**/*.rb
* spec/config/**/*.rb
* spec/controllers/**/*.rb

xref https://gitlab.com/gitlab-org/gitlab-ce/issues/59758

These specs were flaky and only passing after a retry due to how
rspec-retry works.

1. The test with paths that don't exist was returning 200 on the first
  try, then 404 on the second, not because the paths don't exist, but
  because the MR IID didn't change, thus the MR couldn't be found.
  I decided to remove the test entirely since we don't seem to return
  404 for paths that don't exist.
2. The test with a user that cannot view the merge request was failing
  the first time because the project owner wasn't removed with
  `project.team.truncate`.
  Changing the `let(:user)` to `create(:user)` and calling
  `project.add_maintainer(user)` in the `before` block fix the test.

Signed-off-by: Rémy Coutable <remy@rymai.me>

These are used by the EE-only approvers feature

Updates specs to use new rails5 format.

The old format:
`get :show, { some: params }, { some: headers }`

The new format:
`get :show, params: { some: params }, headers: { some: headers }`