server/net: make the user-agent configurable

Fixes #127

server/szurubooru/func/net.py

@@ -1,10 +1,13 @@
 import urllib.request
+from szurubooru import config
 from szurubooru import errors
 
 
 def download(url: str) -> bytes:
     assert url
     request = urllib.request.Request(url)
+    if config.config['user_agent']:
+        request.add_header('User-Agent', config.config['user_agent'])
     request.add_header('Referer', url)
     try:
         with urllib.request.urlopen(request) as handle:

server/szurubooru/tests/func/test_net.py

@@ -1,7 +1,10 @@
 from szurubooru.func import net
 
 
-def test_download():
+def test_download(config_injector):
+    config_injector({
+        'user_agent': None
+    })
     url = 'http://info.cern.ch/hypertext/WWW/TheProject.html'
 
     expected_content = (