Make profile menu full-height

Summary:
Ref T10054. This makes the profile menu full-height. It uses two pieces of dark magic:

• calc(), which allows you to do math in CSS.
• The vh unit, which is CSS for "viewport height".

Apparently this kind of stuff just works now? CSS got good at some point?

Test Plan:

• Page looks correct in Safari, Chrome, Firefox.
• Checked caniuse.com for vh and calc(), saw they're supported?

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10054

Differential Revision: https://secure.phabricator.com/D15017