[less mode] Fix indentation of nested rules

Issue #1801
[less mode] Remove console.log
Better lint icons and in fewer bytes

Original error & warning icons had jagged edges due to being GIFs
This made them look quite poor/low quality on dark BG'd themes
So, new, better looking icons added which have been run thru TinyPNG for
compression
Result is fewer bytes than original and they look good on dark BG's too
Don't mutate options.gutters in setGuttersForLineNumbers

Closes #1807
Closes #1808
Prevent corrupted measurements due to vanishing/appearing scrollbar

Issue #1787