enable custom server url input in debug builds

[iamtalhaasghar]

so that we can test app with our test / staging server or latest matrix server. this is currently disabled but we should do something in code like this:

if (app_build == debug){
    // enable custom server url input for debug release
    default_url = 'testing.server.com'
} else{
   // disable custom server url input for production release
   default_url = 'production.server.com'
}