js: Convert _.extend to spread syntax or Object.assign.

This is not always a behavior-preserving translation: _.extend mutates
its first argument.  However, the code does not always appear to have
been written to expect that.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

static/js/blueslip.js

@@ -80,7 +80,7 @@ const reported_errors = new Set();
 const last_report_attempt = new Map();
 
 function report_error(msg, stack, opts) {
-    opts = _.extend({show_ui_msg: false}, opts);
+    opts = { show_ui_msg: false, ...opts };
 
     if (stack === undefined) {
         stack = 'No stacktrace available';