filter: Use "channels" operator for the Filter class.

Update the Filter class to use "channels" as the canoncial operator
for public stream searches and web-public message fetch narrows,
but keep using "streams" for user-facing text and URLs.

When searching, "channels" does not create any suggestions until
a colon is added and then it shows suggestions with "streams". And
when a search string with channel as an operator is entered, then
it is replaced by "streams" as well.

Part of stream to channel rename project.

web/src/hash_parser.ts

@@ -92,6 +92,7 @@ export function is_create_new_stream_narrow(): boolean {
 }
 
 export const allowed_web_public_narrows = [
+    "channels",
     "channel",
     "streams",
     "stream",