UI - hide and show "settle-now" button on checkbox tick

**Detailed Description**
On checkbox tick, calculate the settlement net amount and if net < 0 (thisUser is a net payer rather than receiver) then show the settle now button.

**Actual Behavior**
Steps to reproduce the behavior:
1. Go to '...'
2. Click on '....'
3. Scroll down to '....'

**Expected Behavior**
A clear and concise description of what you expected to happen.

**Environment:**
- Java: [e.g. OpenJDK 11.0.4]
- Node: [e.g. v12.9.1-x86]
- OS: [e.g. Ubuntu 18.04]
- Version: [e.g. 1.0.1]

**Additional Context**
Add any other context about the problem here. Attach any logs here, if applicable.