Add option for user to reset their avatar

[intersecato]

Describe the feature

If a user decide to upload a custom avatar he can no longer go back to the default one (that one generated by mc-heads or other providers), and only the staff could do that.
I therefore propose to add the option to reset the personal avatar.

[supercrafter100]

Because this requires breaking changes in front-end templates, (adding an extra button) i've marked this for a major version change.