ENH: Standardize header buttons

[choldgraf]

This standardizes our header button behavior, to automate more things, and ensure standard structure, style, and behavior. The user-facing API shouldn't change at all, this is all under-the-hood stuff. However, this should make it much less work to create a "pluggable" header infrastructure when the time comes (ref: #470)

Major changes:

• We now have macros that create different kinds of topbar buttons, and we re-use these throughout
• The header button logic now creates a list of dictionaries that define metadata for the header buttons - this is then rendered into the proper HTML at the end
• Updated the style of the header buttons so that it doesn't use alternating background colors
• The timing of the tooltips is a bit more delayed now so they don't pop up immediately
• Rename the "topbar" to the "article header", in line with the basic-ng theme and more specific.
• Had to update (and in some cases simplify) a bunch of test infrastructure to accommodate the more standard buttons

supercedes #425

OLD:

NEW:

[choldgraf]

Will merge this one in so that we can cut a pre-release and give others a chance to try it out. I took another look and don't think there are any regressions, but something might pop up (if there is something, I suspect it'd be the launch button URL generation)