Add documentation about card class

Better style for challenges

Fix background for lessons

There's a note in
https://github.com/swcarpentry/lesson-template/pull/182
explaining why we need to fix style for panel heading.

This makes some things look nicer (e.g. adds gradients, etc).

Update CSS to go with the SWC site template update

Fix: styles for tables in SQL lessons

Now the style is matching closely `.table-bordered` from Bootstrap.
I decided to leave the header row background blank because it was too
close in color to `#F9F9F9` - our new rule for even rows in the table.

I also increased horizontal padding by 5px on both sides, because now
that we have vertical borders it was too cramped.

Additionaly I introduced borders for every cell in the table.

Previously: table cells had no padding, which caused a visual collapse
(cell content was too close to cell neighbors). Table header rows had
dark blue background and black letters.  Tables had no space between
them and following paragraphs.

Changes:
* added 5px padding to cell contents
* changed header row background to gray-ish (I considered switching text
  color to white, but it didn't look good)
* 15px margin on the bottom of the tables (similarly to bottom margin
  used by paragraph style).

add missing bracket

Upgrade bootstrap

Conflicts:
	css/swc-bootstrap.css
	css/swc.css

Join CSS from site and lessons

As per swcarpentry/lesson-template#140, but only the core CSS
changes.

-   Remove extra space before title of boxes
-   Remove background color of boxes when printing
-   Add "Input", "Output" and "Error" tips for code blocks
    when printing
-   Avoid duplicate lines

and "learning objectives" box.