Add field for sponsors' LinkedIn URL in sponorship application/sponsor profile

[loren-c]

Is your feature request related to a problem? Please describe.
We've begun sometimes acknowledging sponsors on LinkedIn, and it's helpful to have them supply the URL of the correct account to tag when we do.

Describe the solution you'd like
We'd like an optional field in the application form requesting their LinkedIn URL, matching the Twitter handle field. It should also be editable in their sponsor profile.

Additional context
Suggested name for the field:
"LinkedIn page URL"

Screenshot of sponsorship application form with additions in red:

Screenshot of example sponsor profile with additions in red:

@ewdurbin