get_device_browser: Fix Opera detection.

Starting from version 15, Opera uses 'OPR/' in its user agent string.
We need to move Opera detection before Chrome detection because
Opera's user agent string satisfies Chrome conditions as well.
Ref: https://dev.opera.com/blog/opera-user-agent-strings-opera-15-and-beyond/

zerver/signals.py

@@ -15,6 +15,8 @@ def get_device_browser(user_agent):
     user_agent = user_agent.lower()
     if "edge" in user_agent:
         return "Edge"
+    elif "opera" in user_agent or "opr/" in user_agent:
+        return "Opera"
     elif "chrome" in user_agent and "chromium" not in user_agent:
         return 'Chrome'
     elif "firefox" in user_agent and "seamonkey" not in user_agent and "chrome" not in user_agent:
@@ -23,8 +25,6 @@ def get_device_browser(user_agent):
         return "Chromium"
     elif "safari" in user_agent and "chrome" not in user_agent and "chromium" not in user_agent:
         return "Safari"
-    elif "opera" in user_agent:
-        return "Opera"
     elif "msie" in user_agent or "trident" in user_agent:
         return "Internet Explorer"
     elif "zulip" in user_agent:

zerver/tests/test_new_users.py

@@ -89,6 +89,9 @@ class TestBrowserAndOsUserAgentStrings(ZulipTestCase):
                 'Version/10.0.2 Safari/602.3.12', 'Safari', 'MacOS'),
             ('ZulipAndroid/1.0', 'Zulip', 'Android'),
             ('ZulipMobile/1.0', 'Zulip', None),
+            ('Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 '
+             '(KHTML, like Gecko) Chrome/28.0.1500.52 Safari/537.36 '
+             'OPR/15.0.1147.100', 'Opera', 'Windows'),
             ('Mozilla/5.0 (Windows NT 10.0; <64-bit tags>) AppleWebKit/'
              '<WebKit Rev> (KHTML, like Gecko) Chrome/<Chrome Rev> Safari'
              '/<WebKit Rev> Edge/<EdgeHTML Rev>.'