Introduce avatar and banner and fetch Gravatar to fill avatar during registration

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

typo

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Rename avatar to avatar_url, same with header.

Add a comment to explain why the tweak with HTTPoison and TLS1.2

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Rename avatar to avatar_url

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

rename old avatar properties in front-end to avatar_url

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

fix change gravatar from ?d= to ?default=

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

reorganize aliases and imports

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

set avatar url only when gravatar exists, add a test for that case

Signed-off-by: Thomas Citharel <tcit@tcit.fr>