fix(htaccess): check for mod_headers before using Header directive
feature(grid): Implement elgg-grid and make it available
feature(elgg-width): add elgg-width-full

This is useful for creating full-width buttons, or for
forcing one column in a grid to maximize available space.
fix(header): more robust header styles

buttons should really only be 2ems. 2.5 looks a little silly.
However, this was making the header too thin, so the solution
was to put wrappers around the header cells and middle-align
the contents. Using <label> as the wrapper makes it so you can
click anywhere in the cell and the button will get clicked,
even though the button doesn't visually take up the whole height
of the header.

Finally, the triple-dot "more" menu wasn't displaying inline.
It should now show up at the right side of the header, as it was
meant to.
feature(login): simplify login form

The login form is moving toward a unified sign-in/sign-up form that
just asks for the email up front and nothing else. This is meant
to help focus users and open up the possibility of deferring to
third-party login systems if those are available, without ever
presenting a password field to users.