bootstrap_typeahead: Use fixed position for typeaheads.

Since the reference for these typeaheads is has a fixed position
unlike say message controls which move on scroll and need to be
repositioned, we can use `fixed` position for typeaheads too so
there is no jumps when user is trying to scroll when a typeahead
is active.

This basically replaces the commit where where we remove `fixed`
as an argument to `Typeahead` library. We can add it later if
any of our typeaheads don't need to be fixed positioned.

web/src/bootstrap_typeahead.ts

@@ -347,6 +347,7 @@ export class Typeahead<ItemType extends string | object> {
             theme: "popover-menu",
             placement: this.dropup ? "top-start" : "bottom-start",
             popperOptions: {
+                strategy: "fixed",
                 modifiers: [
                     {
                         // This will only work if there is enough space on the fallback placement, otherwise