Do not allow users to change the sub-domains

**Problem**
To avoid mis-uses of the free domain (`authgear.cloud`) we provide for free users, we shall further restrict it by disallow of choosing or changing the domain name for free plan.

**Appetite**
2 weeks

**Solution**
We will:

1. decouple the project name and free subdomain
2. do not allow changing the sub-domain.
3. Add an "Project Setting" page in portal.


Hence for the above screen, we shall change it to two fields:
```markdown
# Create a new Project

**What is the name of your project?**
(text input with any name users used to describe the project)

**This will be a temporary free URL for your authentication end point**
(show the URL of `https://[random human memorizable string].authgear.cloud`, cannot be changed)
```
Nav in portal:

```
> Project
Settings (change project name, show the URL)
Team (Portal Admins)
Billing
Custom Email Provider
```

**Rabbit holes**

**No-goes**
Functionality or use cases we intentionally aren’t covering to fit the appetite

Start by tracing project creation, free subdomain generation, and the portal navigation described in the issue. Compare the current project-name flow with the proposed Project Settings, Team, Billing, and Custom Email Provider pages; done means project names can change independently while the generated free URL remains fixed for free plans.

Written by the indexing model from the issue text.