Fix `UnicodeDecodeError` exception under Python3

Fix the following exception reported in the startup logs:
```
galaxy.webhooks ERROR 2019-01-15 11:31:18,095 'ascii' codec can't decode byte 0xef in position 8291: ordinal not in range(128)
Traceback (most recent call last):
  File "/galaxy/lib/galaxy/webhooks/__init__.py", line 62, in load_webhooks
    self.load_webhook_from_config(directory, config_file_path)
  File "/galaxy/lib/galaxy/webhooks/__init__.py", line 96, in load_webhook_from_config
    webhook.script = file.read()
  File "/galaxy_venv3/lib/python3.5/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xef in position 8291: ordinal not in range(128)
```

Also rename variables from `file` (Python2 standard library function and
class) to `fh`.

lib/galaxy/web/framework/base.py

@@ -499,12 +499,12 @@ def send_file(start_response, trans, body):
     return body
 
 
-def iterate_file(file):
+def iterate_file(fh):
     """
     Progressively return chunks from `file`.
     """
     while 1:
-        chunk = file.read(CHUNK_SIZE)
+        chunk = fh.read(CHUNK_SIZE)
         if not chunk:
             break
         yield chunk

lib/galaxy/webhooks/__init__.py

@@ -1,6 +1,7 @@
 """
 This module manages loading of Galaxy webhooks.
 """
+import io
 import logging
 import os
 
@@ -64,8 +65,8 @@ class WebhooksRegistry(object):
                     log.exception(e)
 
     def load_webhook_from_config(self, webhook_dir, config_file_path):
-        with open(config_file_path) as file:
-            config = yaml.safe_load(file)
+        with open(config_file_path) as fh:
+            config = yaml.safe_load(fh)
 
         weight = config.get('weight', 1)
         if weight < 1:
@@ -83,8 +84,8 @@ class WebhooksRegistry(object):
         # single file
         try:
             styles_file = os.path.join(webhook_dir, 'styles.css')
-            with open(styles_file, 'r') as file:
-                webhook.styles = file.read().replace('\n', '')
+            with open(styles_file, 'r') as fh:
+                webhook.styles = fh.read().replace('\n', '')
         except IOError:
             pass
 
@@ -92,8 +93,8 @@ class WebhooksRegistry(object):
         # single file
         try:
             script_file = os.path.join(webhook_dir, 'script.js')
-            with open(script_file, 'r') as file:
-                webhook.script = file.read()
+            with io.open(script_file, 'r', encoding='utf-8') as fh:
+                webhook.script = fh.read()
         except IOError:
             pass