fix(semantic): improve HTML semantics across templates

[miaulalala]

• Replace navbar-default class on <footer> with site-footer
• Extract filter form out of <ul class="nav-tabs"> into its own sibling element
• Convert .app-meta <div> to <aside aria-label="App information"> on detail page
• Merge duplicate {% if request.user.is_authenticated %} blocks in nav.html

[edward-ly]

Everything looks good apart from one breaking change: while it might make sense to add more visual separation the sort and filter options, there seems to be an issue with the flex layout for the sorting options on smaller screens.

Here's what the options row look like on the master branch.

And here's what the PR changes look like on the same screen. The rating sort is not accessible anymore, in this case.

Make sure the options row remain responsive to the screen width (preferably by restoring the wrapping behavior, but if you have a better solution, I'm all ears).

[edward-ly]

Hm, at certain screen widths (around 993 to 1998 px), there is quite a bit of empty space between the sort and filter options, to the point where it might look better if the filter options were collapsed into the end of the second line rather than be on its own line, in this case. Would you agree, or is there a reason why this layout might be preferred?