Add instructional message to step blocks.

[Kelketek]

To test:

1. Create a step builder block.
2. Add a step to the block.
3. Edit the block. Fill in the message section
4. Add a problem to the block.
5. Save, answer the problem, submit, see new popup thingy. Click anywhere and it should dismiss.

[Kelketek]

@itsjeyd

[itsjeyd]

@Kelketek I tested and the feature is working well overall (love the box-shadow :)).

There are a couple of minor glitches:

1. The fact that the background of the message is transparent can make some areas of it a little harder to read; this mostly affects longer messages. Example screenshots:

   
   

2. If I submit and then click "Next step" without explicitly dismissing the message, the message is visible when I restart Step Builder via "Try again".

Code looks good, just had a couple of minor comments.

[itsjeyd]

@Kelketek This is missing a semicolon.

[Kelketek]

@itsjeyd Ready for another look.

[itsjeyd]

@Kelketek 👍 once you fix the failing build (it's just a leftover blank line at the end of test_step_builder.py).