popovers: Use fixed position for all bootstrap popovers.

`absolute` position elements in message feed can go
offscreen since `html` is now scrollable.

web/src/emoji_picker.js

@@ -649,6 +649,7 @@ export function build_emoji_popover($elt, id) {
         content: generate_emoji_picker_content(id),
         html: true,
         trigger: "manual",
+        fixed: true,
     });
     $elt.popover("show");
 

web/src/giphy.js

@@ -243,6 +243,7 @@ export function initialize() {
         $active_popover_element.popover({
             animation: true,
             placement: get_popover_placement(),
+            fixed: true,
             html: true,
             trigger: "manual",
             template: get_popover_content(),