update gemfile to use byebug for ruby 2.0 since debugger is broken in 2.0

test plan: `bundle install` runs without breaking. `bundle exec jekyll build` also does the same
add notification icon, bump minor version, rebuild all fonts

summary: adds in the squared-off comments icon that we used to use for notifications and bumps to 3.5.5

also marks some glyphs as "to be deleted" in a subsequent version

test plan: icon is present when running jekyl build
rebuilt static site with notification icon