Download fonts at app startup instead

We can't bake the font into the Docker image as such, because we mount
the volumes which blows away anything we have in the app tree
beforehand.

We could stash it somewhere in the image and then copy it from there on
app startup or something, but at that point we might as well just
download it as part of the app startup.

bookwyrm/apps.py

@@ -0,0 +1,35 @@
+import os
+import urllib
+import logging
+
+from django.apps import AppConfig
+
+from bookwyrm import settings
+
+logger = logging.getLogger(__name__)
+
+def download_file(url, destination):
+    try:
+        stream = urllib.request.urlopen(url)
+        with open(destination, "b+w") as f:
+            f.write(stream.read())
+    except (urllib.error.HTTPError, urllib.error.URLError):
+        logger.error("Failed to download file %s", url)
+
+
+class BookwyrmConfig(AppConfig):
+    name = "bookwyrm"
+    verbose_name = "BookWyrm"
+
+    def ready(self):
+        if settings.ENABLE_PREVIEW_IMAGES and settings.FONTS:
+            # Download any fonts that we don't have yet
+            logger.debug("Downloading fonts..")
+            for name, config in settings.FONTS.items():
+                font_path = os.path.join(
+                    settings.FONT_DIR, config["directory"], config["filename"]
+                )
+
+                if "url" in config and not os.path.exists(font_path):
+                    logger.info("Just a sec, downloading %s", name)
+                    download_file(config["url"], font_path)