Implement Snowplow tracking for pricing page A/B test

Utilize the [A/B testing framework](https://gitlab.com/groups/gitlab-com/-/epics/290) for our pricing page test.

MR: https://gitlab.com/gitlab-com/www-gitlab-com/-/merge_requests/44494
Parent issue: https://gitlab.com/gitlab-org/growth/team-tasks/-/issues/74

I've setup the following snowplow events:

Control:

<details>
  <summary markdown="span">On pageload</summary>
triggers the event:

| property | value |
|----------|-------|
| category | `pricing_page` |
| action | `page_view` |
| label | empty |
| se_property | `control` |
| value | empty |
</details>
<details>
  <summary markdown="span">Clicking on the `Sign Up` / `Install` buttons</summary>
![Screen_Shot_2020-04-03_at_14.10.14](/uploads/c73c9d0c2e23ad6fed28a9991003118a/Screen_Shot_2020-04-03_at_14.10.14.png)
![Screen_Shot_2020-04-03_at_14.16.06](/uploads/65bd77f49750748d9df9e75703b1199d/Screen_Shot_2020-04-03_at_14.16.06.png)

triggers these events:

| property | value |
|----------|-------|
| category | `pricing_page` |
| action | `cta_click` |
| label | `free_saas` or `free_self_managed` |
| se_property | `control` |
| value | empty |
</details>
<details>
  <summary markdown="span">Clicking on the `Buy Now` button</summary>
![Screen_Shot_2020-04-03_at_14.10.19](/uploads/b5fc0aca429f044d9d0c48b3961eac3c/Screen_Shot_2020-04-03_at_14.10.19.png)

triggers these events:

| property | value |
|----------|-------|
| category | `pricing_page` |
| action | `cta_click` |
| label | {{plan_id}} |
| se_property | `control` |
| value | empty |
</details>

Test:

<details>
  <summary markdown="span">On pageload</summary>
triggers the event:

| property | value |
|----------|-------|
| category | `pricing_page` |
| action | `page_view` |
| label | empty |
| se_property | `group` |
| value | empty |
</details>
<details>
  <summary markdown="span">Clicking on the `Start now` / `Buy now` buttons to open the modal</summary>
![Screen_Shot_2020-04-03_at_13.21.00](/uploads/31bd8654e335a2b7830482b69726b274/Screen_Shot_2020-04-03_at_13.21.00.png)

triggers these events:

| property | value |
|----------|-------|
| category | `pricing_page` |
| action | `open_modal` |
| label | `free`, `bronze-starter`, `silver-premium` or `gold-ultimate` |
| se_property | `group` |
| value | empty |
</details>
<details>
  <summary markdown="span">Clicking on the `Sign-up for free` / `Install GitLab for free` buttons from the modal</summary>
![Screen_Shot_2020-04-03_at_13.35.03](/uploads/598c09322c7d49197ee3b39e925d9573/Screen_Shot_2020-04-03_at_13.35.03.png)

triggers these events:

| property | value |
|----------|-------|
| category | `pricing_page` |
| action | `cta_click` |
| label | `free_saas` or `free_self_managed` |
| se_property | `group` |
| value | empty |

And `test=jtbd` is added as a URL parameter on the links.
</details>
<details>
  <summary markdown="span">Clicking on the `Purchase SaaS` / `Purchase Self-managed` buttons from the modal</summary>
![Screen_Shot_2020-04-03_at_13.31.57](/uploads/e52a234a68be955fdbd0973f262f44d6/Screen_Shot_2020-04-03_at_13.31.57.png)

triggers these events:

| property | value |
|----------|-------|
| category | `pricing_page` |
| action | `cta_click` |
| label | {{plan_id}} |
| se_property | `group` |
| value | empty |

And `test=jtbd` is added as a URL parameter on the links.
</details>

Story points: 3.0