Web Monetization: there should be validation of the wallet address

[tuxpiper]

Describe the bug
There is no validation on what is provided to the "Wallet Address" input field of the Donation feature configuration.

As a result, deployers may be misled to believe they have correctly configured their wallet, but in reality, the interledger protocol doesn't have the necessary information to operate, and web monetization is not initialized in visitors' browsers. The deployer receives no donations.

Where was the bug observed
Deployments with donation feature enabled in staging and production.

To Reproduce
Steps to reproduce the behavior:

1. Log in as an admin
2. Go to Settings > Donation
3. Under "Wallet Address" enter any text
4. Click on "Enable Monetization"

Expected behavior
The client and the server should check that users' input conforms to the Interledger Protocol specification Payment Pointer syntax

Is there a workaround? What is it?
n/a

Screenshots

URL / Environment where this happened
ushahidi.io - production

[helen-ndip]

@Angamanga @tuxpiper can I work on this ?

[Angamanga]

@helen-ndip I have assigned you to ushahidi/ussd-engine#48

[Angamanga]

@yuktea Assigning this to you!