Added probot bots

[luisivan]

• Adds https://probot.github.io/apps/welcome to welcome people who open issues and PRs and congratulates them on their first PR getting merged
• Adds https://probot.github.io/apps/stale to remind people with issues opened for 6 months, and PRs opened for a month without activity. Closes if there's no reply for a week
• Adds https://probot.github.io/apps/release-drafter to automatically draft release notes by using commit history
• Adds https://probot.github.io/apps/auto-assign to auto-assign @sohkai @facuspagnuolo and @izqui as reviewers for PRs

[coveralls]

Coverage remained the same at 99.542% when pulling 60ee48a on probot-bots into d10d195 on dev.

[sohkai]

@luisivan I've just removed the stablebot configuration for this repo. Given the nature of how we work on this repo (often times there are longer-term issues and PRs open), it's not a great fit and would likely get annoying.

[welcome]

Congrats on merging your first pull request! Aragon is proud of you 🦅