Allow top_links to wrap if too many for space

[will-moore]

Fixes #611.

If we have too many top-links, the header wraps to another line.

NB: the width of the top-links list is slightly hard-coded to be 550px less than the width of the page.
Probably best to use flex layout for the header row to be more flexible (e.g. size of logo and login etc).

[joshmoore]

How hard would it be for this to become a drop-down menu?

[will-moore]

@joshmoore It's doable. But under what conditions?

• All links are always shown in a drop-down menu?
• Links above a certain count go into a More links... dropdown menu?
• Or, it depends on the available space (screen size)? - bit more work.

NB: adding "flex" layout to body causes layout to change on login page:

[joshmoore]

I'd personally prefer any of the options you mentioned over the wrapping behavior.