Add $sub_url to tutor_dashboard_url filter

[bernhardkaindl]

@shewa12 @sifat009, on behalf of @w3guy:

Add $sub_url to tutor_dashboard_url filter

@w3guy said about this submission at https://github.com/themeum/tutor/pull/496#issuecomment-1661586546:

"This PR is important to us though":

I am unable to create a PR against the dev branch. I don't have such permission. Here's my fork https://github.com/w3guy/tutor/commit/40bc7cc3b5c4ccbe0762e5cbf4c101881c595f59

This is the resubmit @w3guy asked for, thanks!

[bernhardkaindl]

@shewa12 please merge, thanks!

[w3guy]

@shewa12 why was this closed?

[bernhardkaindl]

@shewa12

It would be more friendly if you'd describe why you close PRs.

If you do not accept community contributions, please say so in the README.