mailing_lists.md: minor grammar fixes
About: say a word on the possibility to edit on Github
Typo correction
Merge pull request #384 from bluddy/patch-1

Typo correction
Added Esper to list of companies, removed mention of Skydeck (defunct),
updated Mr. Number.
Add a bar to the left of solutions

This enables to spot them better when they are open.
99problems: Add remarks explaining code choices