Add Expression

Authenticated users can add expressions.
There should be a button in the main layout nav to add an expression, or as a menu item off of the username.

Add page should capture details of the expression.
Page should attempt to avoid allowing evil expressions. See #13.

Start by tracing the main layout nav and the authenticated user flow to locate the add page entry point. Read issue #13 before defining how the page should capture expression details and avoid evil expressions; done means authenticated users can reach the page, submit expression details, and have unsafe expressions rejected.

Written by the indexing model from the issue text.