Number of units field accepts any number of units and ends hanging the browser

[Diogo Matsubara]

Steps to reproduce:
1. Open https://ec2-23-20-230-72.compute-1.amazonaws.com
2. Search for a charm in the side bar (e.g. mysql)
3. Click on Add
4. Fill in the number of units field a unreasonable number (e.g. 99999999999999999999999)
5. Click Confirm

What happens:

The browser hangs and you start seeing the browser's java script not responding notifications

[Artur Tyloch (artur-tyloch)]

it hangs also for more reasonable numbers like: 1000 or 10000
It is bad user experience to lack some even very simple validation of input and limit max number or warn user.