Ability to rename the "Start page" to anything that makes sense to the rest of the form

## Description

The "Start page" or "Page de départ" in French is named by default.

As this name now shows up in the review page and in the form in progress file as well as the copy of answers at the end after submitting the form, we might want to consider allowing clients to rename this to match the rest of the flow of their form.

If subsequent pages have a specific naming convention such as "Section 2: Applicant information, Section 3: Project overview, Section 4: Budget" then it might be nice for the first page to be "Section 1: Eligibility requirements and privacy" or something of the sort that's customizable if desired.

_Discovered when trying to build/replicate an elaborate G&Cs form_

## Steps to reproduce
1. Go to GC Forms
2. Build a multi-page form
3. Name all pages
4. Go to Review page
5. Save answers to device
6. Submit
7. Download a copy of answers
8. Compare the names of all pages

## Details
- **Browser**: Chrome
- **Operating system**: Mac
- **Environment**: Production
- **Language**: English and French
- **Viewport**: Desktop

## Expected behaviour
All page names that are visible to end users filling out forms should be customizable by form builders so that the form is more consistent/cohesive.

## Screenshots or videos

Trace how page names are defined and displayed across the GC Forms builder, the review page, the in-progress form file, and the submitted answers download. Start by reproducing the multi-page flow described in the issue in both English and French. Done means form builders can customize the start page name and that the chosen name appears consistently wherever other page names are shown.

Written by the indexing model from the issue text.