DataLayer Events for SaaS Trial Flow

## Goal
There are some pages within the SaaS Trial Flow without GA event tracking/GTM code snippet. Adding dataLayer events and GTM code will allow us to see the full picture in the user's journey from the marketing site to an ultimate trial. 

For context, I've created a [deck of all possible paths](https://docs.google.com/presentation/d/11PiN-e79QLXaURmNfVemOexYX2_yEjKBziHzHeH4nWE/edit#slide=id.g11b167403ab_0_104) the user may experience.
![Screen_Shot_2022-04-13_at_2.39.17_PM](/uploads/7b4466ce6463f80dc86275421e517c3c/Screen_Shot_2022-04-13_at_2.39.17_PM.png)

## Page(s)

**gitlab.com/-/users/terms**<br/>
Fire on "Accept Terms" button click
```
window.dataLayer = window.dataLayer || [];
dataLayer.push({
 'event': 'saasTrialAcceptTerm'
});
```
<br/>

**gitlab.com/users/sign_in**<br/>
Place OneTrust banner first! Paste this code as high in the `<head>` of the page as possible:
```
<!-- Google Tag Manager -->
<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-NJXWQL');</script>
<!-- End Google Tag Manager -->
```

Additionally, paste this code immediately after the opening `<body>` tag:
```
<!-- Google Tag Manager (noscript) -->
<noscript><iframe src="https://www.googletagmanager.com/ns.html?id=GTM-NJXWQL"
height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>
<!-- End Google Tag Manager (noscript) -->
```
<br/> 

**gitlab.com/users/sign_up/welcome**<br/>
Fire on "Continue" button click
```
window.dataLayer = window.dataLayer || [];
dataLayer.push({
 'event': 'saasTrialWelcome'
});
```
<br/>

**gitlab.com/users/sign_up/welcome/continuous_onboarding_getting_started**<br/>
Fire on "Okay Let's Go" button click
```
window.dataLayer = window.dataLayer || [];
dataLayer.push({
 'event': 'saasTrialContinuousOnboarding'
});
```


## DCI
[DRI, Consulted, Informed](https://about.gitlab.com/handbook/people-group/directly-responsible-individuals/#dri-consulted-informed-dci)

- [ ] DRI: @dennischarukulvanich @tywilliams 
- [ ] Consulted: @jstemmer1 
- [ ] Informed: @mpreuss22 


## Requirements
What are the requirements for this request? Checklist below is an example of common requirements, please check all that apply and adjust as necessary:

- [ ] Copy writing
- [ ] Illustration
- [ ] Custom Graphics
- [ ] Research
- [x] Data / Analytics
- [ ] UX Design
- [x] Engineering

## Progress

1. [x] `gitlab.com/-/users/terms`: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/91936
2. [x] `gitlab.com/users/sign_in`: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/92124
3. [x] `gitlab.com/users/sign_up/welcome`: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/96187
4. [x] `gitlab.com/users/sign_up/welcome/continuous_onboarding_getting_started`: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/96187

Story points: 3.0