Better handling for profile settings URLs

Description

Given there has been some confusion about profile links on whether to include a URL prefix or not (see https://gitlab.com/gitlab-org/gitlab-ce/issues/20681#note_68860044) which can lead to broken URLs, we could include a fixed, immutable input prefix text for some inputs.

Proposal

1. Include https://linkedin.com/in/ prefix on Linkedin input on /profile.
2. Include https://twitter.com/ prefix on Twitter input on /profile.
3. Website input should automatically add http:// unless https:// is specified.

• Alternatively, we could handle better user input.
• In the future we could remove also remove links to social profiles.

URL prefix