Abstracted user avatar fetching away from gravatar

Still uses gravatar as a default.
Updated URL placeholders to follow LDAP format.
Potential breaking config change: `GRAVATAR=false` replaced by `AVATAR_URL=false`
Builds upon #1111

.env.example

@@ -60,13 +60,13 @@ GITLAB_BASE_URI=false
 DISCORD_APP_ID=false
 DISCORD_APP_SECRET=false
 
-# External services such as Gravatar and Draw.IO
+
+# Disable default services such as Gravatar and Draw.IO
 DISABLE_EXTERNAL_SERVICES=false
-# Default GRAVATAR_URL set to Gravatar service
-GRAVATAR_URL=false
-# To use a different service to get user's avatar like libravatar
-# Possible placeholders: %{hash} %{size} %{email}
-#GRAVATAR_URL=https://seccdn.libravatar.org/avatar/%{hash}?s=%{size}&d=identicon
+# Use custom avatar service, Sets fetch URL
+# Possible placeholders: ${hash} ${size} ${email}
+# If set, Avatars will be fetched regardless of DISABLE_EXTERNAL_SERVICES option.
+# AVATAR_URL=https://seccdn.libravatar.org/avatar/${hash}?s=${size}&d=identicon
 
 # LDAP Settings
 LDAP_SERVER=false