Add link to opened pull request

[nijel]

Describe the problem

When I go to the Weblate's repository maintenance, there is no way to go the pull/merge request it opened.

Solution brainstorm

We know the pull request information when creating it, so it's "just" a matter of storing it and exposing the link in the repository management.

Describe alternatives you have considered

No response

Screenshots

No response

Additional context

No response