Add newsletter sign-up globally to footer

<!--

This template is for requesting work to an existing webpage  [marketing website](https://about.gitlab.com/) , or logged out gitlab.com.

DO NOT use this for:
New webpages, docs.gitlab.com, the GitLab product (In app), the handbook or other items.

For information regarding working on the about website please visit the [Brand and Digital Handbook](https://about.gitlab.com/handbook/marketing/inbound-marketing/digital-experience/)

TITLE THIS ISSUE: Page Update: [Your Relevant Title]
-->
## Overview

## Triage (REQUIRED)
 
**This issue fulfills one of these goals:** (Requestor check one)
- [ ] Traffic
- [x] Supports Inquries
- [ ] Conversion 
- [ ] Enable unlocks for conversion and traffic later (example CMS implementation)
   - `Add details here (required if this box is checked)` 
- [ ] Cross-functional support 
   - `Link team to OKR (required if this box is checked)` 
   
#### Briefly describe your request
What is going to be added/updated/removed, etc.

Design and add the newsletter sign-up globally to the about.gitlab.com footer 

#### Describe why this update is being requested:
What problem(s) are you trying to solve? What are the goals? Example: Users can't find what they're looking for.

Assist with Inbound Marketing Inquiries goal

## Project Management

#### Please identify the DRI, stakeholders, subject matter experts, and their roles
Who should we be asking about questions and reviews?

* A directly responsible individual (DRI) is the singular person leading this project. We will ask them for clarifications, decisions, facilitation, or other assistance.
* A stakeholder is a person who represents an entity that the project impacts. We might ask about their needs, request their feedback, or inform them when the project is complete.
* Subject matter experts (SME) will be asked questions related to their expertise. Example subjects include design, development, copy, analytics, seo, social, ops, handbook, infrastructure, technologies like Marketo, etc.
* You may insert or remove as many lines as needed to list the relevant people involved with the project and their roles.

|Name|Role|
|------|------|
| @mpreuss22  | DRI |
| @vsilverthorne   | Content SME|
| `@user2` | UX design |
| `@user3` | Development |
| @bmatturro  | Content stakeholder|
| @dmor  | Stakeholder|


#### What is the timeframe AND WHY?
Dates without reason are difficult to prioritize. Without enough lead time we will be unable to meet your deadline. Example: Our due date is XXXX-XX-XX. We want to launch this page before a trade show happens on YYYY-YY-YY. We need a week to coordinate press before the event.

*If your ask requires multiple delivery deadlines, consider creating an epic with an issue for each deadline.*

Align with Navigation updates if the footer is included, if not let's aim for mid February

## Project Deliverables

* [ ] Add newsletter sign-up form to global footer

## Related Documents/Information
Link and explain all related documents, websites, assets, logos etc. NOTE: The web team will not be able to meet deadlines unless everything is provided. [Images should follow our guidelines](https://about.gitlab.com/handbook/marketing/inbound-marketing/digital-experience/image-guidelines/).

* Newsletter CTA: @vsilverthorne to provide
* Newsletter form URL: @vsilverthorne to provide 

Hashicorp as example: 
![image](/uploads/cda883ba80614bec1ddf70b0d6970b9b/image.png)
_____________
_____________
_____________

## Requestor Checklist

* [x] All sections have been filled out.
* [ ] All media and relevant documents have been linked or added.
* [ ] A due date and rationale for the due date has been added.
* [ ] Your team labels have been added.
* [ ] Any issues blocking this have been related as blocking.
* [ ] The supplied copy is not a placeholder and is expected to be the final copy, ready for development.

Story points: 4.0