Update link in waitlisted email from /partners to /users/my_library

[Mortal303]

Update link from https://wikipedialibrary.wmflabs.org/partners/ to https://wikipedialibrary.wmflabs.org/users/my_library/
in (TWLight/emails/tasks.py) the email users receive when applying to a waitlisted resource.

Bug:T302498

Description

Describe your changes in detail following the commit message guidelines

Rationale

Phabricator Ticket

How Has This Been Tested?

Screenshots of your changes (if appropriate):

Types of changes

What types of changes does your code introduce? Add an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Minor change (fix a typo, add a translation tag, add section to README, etc.)

[suecarmol]

Thank you for working on this. LGTM!