Custom safe apps don't load unless they contain a trailing slash

[usame-algan]

Bug description

When adding a custom safe app, the trailing slash is removed. Without a trailing slash the safe app doesn't open unless it is added back manually in the URL

Related issue: #732

Steps to reproduce

1. Go to safe apps
2. Add a custom safe app
3. Copy a safe app url with a trailing slash
4. Try to open that safe app afterwards

Expected result

The safe app opens

Obtained result

The safe app is loading indefinitely