Prevent rate limit errors in wayback machine API (#339)

The Wayback Machine Save API only allows a limited number of requests within a timespan. This introduces several changes to avoid rate limit errors:
- There will be max. 1 attempt to create a new snapshot
- If a new snapshot could not be created, then attempt to use the latest existing snapshot
- Bulk snapshot updates (bookmark import, load missing snapshots after login) will only attempt to load the latest snapshot instead of creating new ones

bookmarks/services/wayback.py

@@ -0,0 +1,40 @@
+import time
+from typing import Dict
+
+import waybackpy
+import waybackpy.utils
+from waybackpy.exceptions import NoCDXRecordFound
+
+
+class CustomWaybackMachineCDXServerAPI(waybackpy.WaybackMachineCDXServerAPI):
+    """
+    Customized WaybackMachineCDXServerAPI to work around some issues with retrieving the newest snapshot.
+    See https://github.com/akamhy/waybackpy/issues/176
+    """
+
+    def newest(self):
+        unix_timestamp = int(time.time())
+        self.closest = waybackpy.utils.unix_timestamp_to_wayback_timestamp(unix_timestamp)
+        self.sort = 'closest'
+        self.limit = -5
+
+        newest_snapshot = None
+        for snapshot in self.snapshots():
+            newest_snapshot = snapshot
+            break
+
+        if not newest_snapshot:
+            raise NoCDXRecordFound(
+                "Wayback Machine's CDX server did not return any records "
+                + "for the query. The URL may not have any archives "
+                + " on the Wayback Machine or the URL may have been recently "
+                + "archived and is still not available on the CDX server."
+            )
+
+        return newest_snapshot
+
+    def add_payload(self, payload: Dict[str, str]) -> None:
+        super().add_payload(payload)
+        # Set fastLatest query param, as we are only using this API to get the latest snapshot and using fastLatest
+        # makes searching for latest snapshots faster
+        payload['fastLatest'] = 'true'