feat(upload): avatar and calendar cover via local disk storage.

POST /v1/user/me/avatar and /v1/calendars/:id/cover; GET /v1/media;
UPLOAD_DIR on eventhub-data volume; MIME/size limits.
Refs EventHub/EventHubBack#71
Refs EventHub/EventHubSpec#8

docker/erlang/Dockerfile.runtime

@@ -7,7 +7,7 @@ FROM alpine:${ALPINE_VERSION}
 RUN apk add --no-cache \
     openssl libstdc++ libgcc ncurses-libs libsodium \
     file \
-    && mkdir -p /app/data \
-    && chmod 777 /app/data
+    && mkdir -p /app/data /app/data/uploads \
+    && chmod 777 /app/data /app/data/uploads
 
 WORKDIR /app