Fixing Notification Messages Format

[hylstonnb]

Follow-up work of PR948

This PR contains the following changes:

• Changed all the notification message * symbols to bullet point symbol to be compatible with other platforms such as slack
• Updated the code to use a same bullet point constant (if changes are needed we can now change it in a single place).
• Added an empty line after the baseMessage. This is helpful when there are more some custom text to show after the base message, so this text will be append after an empty line.
• Unit tests updated accordingly

[aurambaj]

when you're done with your change, would you squashing/rebasing (git rebase -i HEAD~4 or the applicable number) to just have meaningful commit (here a single commit is probably enough)

[hylstonnb]

closing this one as it is going to be replaced by https://github.com/box/mojito/pull/950