Show solutions for all completed exercises

A couple of people have requested being able to see the 'official' solution after completing an exercise, so they can see a good way to solve the problem and compare it to their own. Here's one screenshot from slack:


This clearly matters quite a lot based on the effort they went through to work around the missing feature:

> Right now I just wrote my own code and clicked the hints and solution before actually running my code

See also this reddit comment: https://www.reddit.com/r/learnprogramming/comments/qjs3wh/i_built_futurecoder_a_100_free_and_interactive/i1ytrl8/

> so in order for me to backtrack I had to open the page in another browser window and redo all previous sections to get to the one I want to be on

> also to repeat a prior comment. it would be nice to be able to see ur solution before it auto bumps to the next section.

Note that there's a better workaround in the meantime, which is to turn on developer mode in the settings and use the 'reverse step' button. I've thought about making that button always available regardless of developer mode, but it's not clear how often it would be useful outside of this use case, and there are better ways to solve this particular problem.

Somewhat related:

- https://github.com/alexmojaki/futurecoder/issues/91
- https://github.com/alexmojaki/futurecoder/issues/84
- #85