ENH: Stimulate users to add an Avatar

[Timo-Breumelhof]

Is your feature request related to a problem?

Please describe.
I think a Forums looks much more alive when all users have an Avatar.

Describe the solution you'd like

I would like an option to stimulate users to upload an avatar.
Like this maybe?

So that would mean 2 things:

A. A way to set the default Avatar (I guess from the settings?)
This would also allow an MTB Forum to use this as the default:

B. An option to add a button to "Edit profile" for the user (only visible for that user)

C. Maybe we could also add an image "a user without an image" sees if they visit the forums? (not public)

Describe alternatives you've considered

Adding a weird Photo by default..

Additional context

Related to #347