register: Handle "Pronouns" type fields for older mobile clients.

Mobile clients older than v27.192 do not support PRONOUNS type
custom profile fields, so we instead change the type of it to
SHORT_TEXT in the data sent with register response and also in
the events sent to those clients.

zerver/lib/compatibility.py

@@ -138,3 +138,24 @@ def is_unsupported_browser(user_agent: str) -> Tuple[bool, Optional[str]]:
     if browser_name == "Internet Explorer":
         return (True, browser_name)
     return (False, browser_name)
+
+
+def is_pronouns_field_type_supported(user_agent_str: str) -> bool:
+    # In order to avoid users having a bad experience with these
+    # custom profile fields disappearing after applying migration
+    # 0421_migrate_pronouns_custom_profile_fields, we provide this
+    # compatibility shim to show such custom profile fields as
+    # SHORT_TEXT to older mobile app clients.
+    #
+    # TODO/compatibility(7.0): Because this is a relatively minor
+    # detail, we can remove this compatibility hack once most users
+    # have upgraded to a sufficiently new mobile client.
+    user_agent = parse_user_agent(user_agent_str)
+    if user_agent["name"] != "ZulipMobile":
+        return True
+
+    FIRST_VERSION_TO_SUPPORT_PRONOUNS_FIELD = "27.192"
+    if version_lt(user_agent["version"], FIRST_VERSION_TO_SUPPORT_PRONOUNS_FIELD):
+        return False
+
+    return True