Refactor usage of this.refs to access form data

[norbertschuler]

Using this.refs to access form data sometimes fails, e.g.

• undefined is not an object (evaluating 'u.refs.targetContainer.refs') #1989 or undefined is not an object (evaluating 'u.refs.targetContainer.refs') #2216
• TypeError in /edit-profile - null is not an object (evaluating 't.password.first') #1998
• undefined is not an object (evaluating 'l.refs.EditUserProfileContainer.refs') #2085
• TypeError in /leaderboard/country - undefined is not an object (evaluating 't.options[t.selectedIndex].value') #2208
• TypeError in /edit-profile - Cannot read property 'first' of null #2524
• TypeError in app/components/Authentication/SignUp/SignUp.native.js:45 - null is not an object (evaluating 'o.signupForm.getValue') #2566
• TypeError in /leaderboard/country - undefined is not an object (evaluating 't.options[t.selectedIndex].value') #2574

Therefore the usage probably should be change towards the solution used at

treecounter-app/app/components/EditUserContribution/index.js

Line 110 in 2b968b0

ref={ref => (this.editTreeForm = ref)}

in the whole codebase!

[norbertschuler]

There is another solution not working everytime which might also get changed: #2509

[norbertschuler]

We should refactor using https://reactjs.org/docs/refs-and-the-dom.html as already done for one component in #2625.