Provide a way to add more seats for Cron and Uptime self-service customers

### Problem Statement

**Problem**

There is no way users can add more seats for Cron and Uptime in the app. Whether the one seat has been reached or needs to add more seats.


### Solution Brainstorm

Maybe add a section for Cron and a section for Uptime here, and to allow add/remove seats

### Product Area

Settings - Subscription

The issue names no files, tests, or entry points. Start by reviewing Settings - Subscription and the existing Cron and Uptime self-service seat handling. Done means users can add and remove seats for both products in the app.

Written by the indexing model from the issue text.