Add microblog_footer.html

[manton]

Adding this issue to put a couple thoughts down in one place. Similar to microblog_head.html, I think we do need a footer version to balance it. This would include the basic HTML that most themes need, like including custom_footer.html and the JS includes.

The reason this seems important to me now is I've been thinking about adding a footers field to plugin.json to include other HTML before the closing </body>. There's not currently a great way to do that unless you just need JS.

(I've also thought about a kind of replacements field that could replace arbitrary HTML tags, not just </body>… But that has performance tradeoffs that I don't think we want to deal with right now.)

We may or may not do this. Just don't want to forget. If we change our mind, I'll close this issue.