settings: Add setting to control how animated images are played.

Previously animated images were automatically played in the
message feed of the web app.

Now that we have still thumbnails available for them, we can add a new
personal setting, "web_animate_image_previews", which controls how the
animated images would be played in the web app message feed -- always
played, on hover, or only in the image viewer.

Fixes #31016.

zerver/views/user_settings.py

@@ -161,6 +161,7 @@ def confirm_email_change(request: HttpRequest, confirmation_key: str) -> HttpRes
 
 emojiset_choices = {emojiset["key"] for emojiset in UserProfile.emojiset_choices()}
 web_home_view_options = ["recent_topics", "inbox", "all_messages"]
+web_animate_image_previews_options = ["always", "on_hover", "never"]
 
 
 def check_settings_values(
@@ -329,6 +330,9 @@ def json_change_settings(
     user_list_style: int | None = REQ(
         json_validator=check_int_in(UserProfile.USER_LIST_STYLE_CHOICES), default=None
     ),
+    web_animate_image_previews: str | None = REQ(
+        str_validator=check_string_in(web_animate_image_previews_options), default=None
+    ),
     email_address_visibility: int | None = REQ(
         json_validator=check_int_in(UserProfile.EMAIL_ADDRESS_VISIBILITY_TYPES), default=None
     ),