Before this change, pages that opt in with `use_responsive_design`
would often have the top nav be real cluttered for logged-in users. (I
think I happened to first test this responsive design without being
logged in on my dev box, oops!) Because the home link and `#userbar`
were absolutely positioned on the page, they would frequently overlap.
Here, I stop doing our old tricks to make the top nav load last on the
page. (This was to get "main content" loading faster, which I think is
a. not as relevant today with more commonly faster connections, and b.
was a bit naive to think that it'd be helpful to have to wait a long
time to _navigate_ if a page is unexpectedly large.)
These tricks used to leave some padding at the top of the `#container`,
which these elements would then visually fill via `position: absolute`
once they load.
Next, I update the CSS (for the responsive design pages only) to use
the new `#main-nav` container to lay them out in Flexbox: all in one
row if possible, or wrapped if needed.
Some designs hide stuff like this into a hamburger menu or such when
the screen gets small. I haven't done that here! No specific reason,
I'm just not sold that it's that much better, or worth the trouble.
I tested this on the following combinations:
1. Logged out, homepage
2. Logged in, homepage
3. Logged out, `/items`
4. Logged in, `/items`
5. Logged out, `/items/89487-Dyeworks-Purple-Haunted-Sky-Background`
6. Logged in, `/items/89487-Dyeworks-Purple-Haunted-Sky-Background`
Hope it's solid! 🤞
We add a new `use_responsive_design` helper, for pages to opt into this
new CSS—mostly just because like… it's *worse* to apply these styles
for pages that don't expect it 😅
And then, I fix up a couple things on the item page (including in the
general items layout) to match!
I'm doing this because the species face picker layout is going to want
some responsive awareness, and I want to be doing that from the start!
