Removed username field from password recovery form

[frankcorneliusmartin]

Fixes #1121

Note that I left in the code that can handle both email and username. I felt like leaving it in in case we had a change or hearth. But let me know if it needs te be removed.

[bartvanb]

This works but it leaves unused code. The typescript file should be adjusted as well to remove the username from the form, and then also the call to the API should be adjusted.

[frankcorneliusmartin]

This wil remain as this is a server response.

• I could not get rid of the if (emailValue) { part... Is there a better way to do it? We know there always will be a string (even an email formatted one now).
• I removed the FormBuilder.group method call as it is deprecated, see https://angular.io/api/forms/FormBuilder#group