Sales engagement in tier upgrade process

### Description

## Current Behaviour
FlowFuse Cloud users currently have the ability to independently change their service tier.

## User Story
Role: Sales Personnel
Need: As a salesperson, I want the opportunity to interact with users and businesses interested in upgrading to a specific tier, such as the Enterprise Tier.
Purpose: This interaction aims to facilitate negotiations for beneficial deals and to gain a deeper understanding of the clients' specific needs and requirements.

## Feature Scope

- Admin-Controlled Upgrade Path: Disable Self-Upgrade: Introduce a feature in the Admin Tier configuration that allows the disabling of the self-upgrade option for users.
- Redirect to Sales Contact: When a user attempts to upgrade, they should be redirected to a Sales contact form instead of the upgrade interface. The button for this action should be labeled "Contact Sales for Upgrade."
- Admin Privilege for Upgrades: Administrators should retain the ability to directly upgrade a team to the desired tier

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate