Quite a big site update

Overview of the changes:

* General clean-up:
  - Reformatting of some bits.
  - Rewriting/updating some of the text (I tried to keep the meaning intact).
  - Various small UI changes.
* Added announcement page for the 28 July meetup.
* Added Pelican (theme) level functionality for announcements (to avoid hard-coding it). I called the theme gpuglican ^_^. It should probably be sent upstream to pelican-bootstrap3.
* Switched the Bootstrap theme to spacelab. It just looks a lot cleaner to me, but feel free to chuck out that change.
* Replaced the hard-coded welcome message with functionality to embed pages in the home page. The welcome message is now included from `home.md`.

I leave the merge of this pull request to someone else (@Kentoseth?), to make sure that at least one other person agrees with the changes.

The generated content was **not** pushed to the `gautengpug.github.io` repo.