Deprecate GitlabPlansHelper if favor of additional attribute in the namespace object

<!-- Please complete the template below as best as you can. Make sure to check if this issue has already been raised by someone else first to avoid duplication. 

For each section below, please add screenshots or links or anything that may help visual learners understand the problem better, even if this takes you an extra minute or two this is a great help to some folks.

https://www.learning-styles-online.com/style/visual-spatial/

-->

### Problem

We use `GitlabPlansHelper` every time we want to pull Gitlab namespaces for a customer:

* To fill a select with available namespaces when purchasing a new subscription;
* To fill a select with all namespaces when a customer wants to change linked namespace;

We are slowly transitioning to Vue.js and using this helper becomes harder because we need more data in a slightly different form. For instance, we want to see if a returned namespace has an expired subscription.

### Proposal

I suggest to deprecate this helper and add a new attribute called `subscription` to the existing `namespace` object.

**Current `namespace` object structure:**

```
                                    "id" => 53,
                                  "name" => "gold",
                                  "path" => "gold",
                                  "kind" => "group",
                             "full_path" => "gold",
                             "parent_id" => nil,
                            "avatar_url" => nil,
                               "web_url" => "http://127.0.0.1:3000/groups/gold",
        "members_count_with_descendants" => 1,
                "billable_members_count" => 1,
                                  "plan" => "silver",
                         "trial_ends_on" => nil,
                                 "trial" => false,
```

**Suggested `namespace` object structure:**

```
                                    "id" => 53,
                                  "name" => "gold",
                                  "path" => "gold",
                                  "kind" => "group",
                             "full_path" => "gold",
                             "parent_id" => nil,
                            "avatar_url" => nil,
                               "web_url" => "http://127.0.0.1:3000/groups/gold",
        "members_count_with_descendants" => 1,
                "billable_members_count" => 1,
                                  "plan" => "silver",
                         "trial_ends_on" => nil, // deprecate this field also and move it under subscription attribute
                                 "trial" => false, // deprecate this field also and move it under subscription attribute
                          "subscription" => {
                     "name" => "A-S00039218",
               "start_date" => Tue, 30 Jun 2020,
                 "end_date" => Wed, 30 Jun 2021,
            "trial_active" => false
        }
```

**NOTE:** I do not suggest removing it completely because we have the code that depends on this helper but stop adding new functionality to it.

### Result

* Removed legacy helper with better-structured data.
* Removed code that is hard to maintain due to its growing complexity.

### Next steps (if any)

### How will we measure success?

Story points: 1.0