read_receipts: Add support for displaying read receipts.

Adds an API endpoint for accessing read receipts for other users, as
well as a modal UI for displaying that information.

Enables the previously merged privacy settings UI for managing whether
a user makes read receipts data available to other users.

Documentation is pending, and we'll likely want to link to the
documentation with help_settings_link once it is complete.

Fixes #3618.

Co-authored-by: Tim Abbott <tabbott@zulip.com>

templates/zerver/api/changelog.md

@@ -22,6 +22,8 @@ format used by the Zulip server that they are interacting with.
 
 **Feature level 137**
 
+* [`GET /messages/{message_id}/read_receipts`](/api/get-read-receipts):
+  Added new endpoint to fetch read receipts for a message.
 * [`POST /register`](/api/register-queue), [`GET
   /events`](/api/get-events), `PATCH /realm`: Added new
   `enable_read_receipts` realm setting.

templates/zerver/help/include/rest-endpoints.md

@@ -14,6 +14,7 @@
 * [Get a message's edit history](/api/get-message-history)
 * [Update personal message flags](/api/update-message-flags)
 * [Mark messages as read in bulk](/api/mark-all-as-read)
+* [Get a message's read receipts](/api/get-read-receipts)
 
 #### Drafts