Extract edit-user-name widget, add avatar updating to it

[blazern]

Extract edit-user-name widget into a EditUserDataWidget, just like langs editing is extracted into UserLangsWidget.
Just like UserLangsWidget, EditUserDataWidget should not send data to the backend itself. It should be only the representative of data added to it, accept callbacks and/or controllers (like TextField does).

When the avatar in EditUserDataWidget is clicked, the app should initiate photo selection process. The primary goal of such selection is to let user select a photo from their phone's gallery, not to take a new picture.

Once the photo is taken, ImageCropPage should immediately be opened. ImageCropPage must force the cropped area to be a square, user should see it's a square.

Once the photo is cropped, it should be downsized (if too big) according to the size figured out in plante-app-team/plante_server#5.

Once the photo is downsized, it should be set as the selected image of EditUserDataWidget.

Maybe it makes sense to create a AvatarSelectionHelper, which would: initiate photo selection, cropping, then would downsize the image.

When user clicks "Done" on the page after the avatar is chosen, the avatar is sent to the backend. The Backend class should support that.

Avatar selection is optional, not mandatory. This should be explicitly mentioned in the UI.