Replaced Gravatar with self-hosted avatars

Avatars are now obtained from Blender ID. They are downloaded from
Blender ID and stored in the users' home project storage.

Avatars can be synced via Celery and triggered from a webhook.

The avatar can be obtained from the current user object in Python, or
via pillar.api.users.avatar.url(user_dict).

Avatars can be shown in the web frontend by:

- an explicit image (like before but with a non-Gravatar URL)
- a Vue.js component `user-avatar`
- a Vue.js component `current-user-avatar`

The latter is the most efficient for the current user, as it uses user
info that's already injected into the webpage (so requires no extra
queries).

src/styles/_user.sass

@@ -101,3 +101,9 @@
         color: $color-success
       &.fail
         color: $color-danger
+
+img.user-avatar
+  border-radius: 1em
+  box-shadow: 0 0 0 0.2em $color-background-light
+  height: 160px
+  width: 160px