Accept full Linkedin URL's on GitLab profile page

Description

Currently on the user page, the form label just says Linkedin So it would be reasonable to fill in just your linkedin_username or the full url like https://www.linkedin.com/in/linkedin_username

But we accept any text and prepend https://www.linkedin.com/in/

This creates two problems:

1. A user might fill in https://www.linkedin.com/in/linkedin_username and not know that it is broken until they try to visit it and notice that it wrongly takes them to https://www.linkedin.com/in/https://www.linkedin.com/in/linkedin_username

2. There is absolutely no way to add a Linkedin Company page like https://www.linkedin.com/company/gitlab-com if there is a company account.

Proposal

Accept a full URL's or linkedin_username and figure out what the user submitted so we can have a working URL either way.