Prefer GitLab username and slug to twitter handle as anchor in team page

In the team.yml file, when our team members have Twitter handle, the handle is used as an anchor to refer to them in the team page. I think it should be the gitlab username or slug instead.

This MR:

• Removes twitter from the anchor of members
• Keeps a data-twitter attr to be able to find members (e.g. external links, from bookmarks). It tries to find members when the team pages loads.
• Fixes an undefined exception and one event handling instance.