use urlsafe_b64endcode

[dtkav]

I'd like to use the cursors in the query string, but they currently aren't url safe, so some of the characters are url-encoded.

The goal of this diff is to remove the need for url-encoding the cursor.

There are three main changes:

1. switch to using base64.urlsafe_b64encode and base64.urlsafe_b64decode
2. strip and re-pad the b64 padding character (=), which would otherwise be url-encoded
3. fall back to b64decode if urlsafe_b64decode fails (for backwards compatibility)

Let me know what you think!

[moggers87]

Rather than messing around with urlsafe_b64encode, wouldn't it make more sense to do search and replace instead? That would avoid the need to do any fallback and I think the code would be more readable too.