Add a Recently Added Entry on the home page

[wkearn]

This PR adds the ABlog extension, which treats the Gallery posts more like a blog. Thus they can have dates and some metadata associated with them, and we can use this metadata in various ways. Here I have introduced a "Recently Added" section, that highlights the most recently added notebook. Other types of things we could do with this:

• Highlight any other notebooks of particular interest
• Use tags to group notebooks together
• Replace the python/matlab directories with tags
• List gallery entries by author
• Make other blog posts or updates either here in the Gallery or elsewhere in the TT3 website.

I have currently added my Roan Plateau notebook and Boris's first GraphFlood notebook (not #43) as "posts". The remaining Python notebooks can also be added as posts.

This is not quite an optimal solution at the moment for reasons discussed below, but I wanted to present it as a possibility for restructuring things a little bit.

You need to add some metadata in the form of a raw reStructuredText cell to the top of the notebook. This is a little tricky for potential contributors because you have to fiddle with the notebook metadata.

I also have not figured out a good way to add this cell to MATLAB Live Scripts. We'll need to do some postprocessing, like we handle images at the moment. I am not sure of a good way to embed the metadata in the Live Script so that we can access it with a filter program.

The post authors are maintained in a directory in conf.py. Every time a new author contributes, they'll need to add themselves to the directory. All of the posts contributed by a given author are collected in an automatically generated page, and the author's homepage is linked, which is rather nice.

The authors and publication date shows up in the left sidebar. I don't know if it is possible to make them appear in the post itself. If we figure that out, we could replace the typical "Authors" section with this information.

[wkearn]

This is what the home page looks like:

[wkearn]

And a post (note the left sidebar):

[wschwanghart]

This is in fact a good idea and I like the layout and structure. However, we should be careful not to introduce any additional complications that prevent users from contributing. To find an intermediate solution, would it be possible to add metadata in some structured txt-file?