Add mergify bot backport functionality

[fedekunze]

Automatically backport PRs to release/0.7.x branch based on PR label

Ref: https://github.com/cosmos/cosmos-sdk/blob/master/.mergify.yml

[fedekunze]

@tomtau @yihuang @thomas-nguy this will ease the backporting process for you guys

[thomas-nguy]

just by curiousity, how does the bot handle merge conflict?

[tomtau]

@thomas-nguy AFAIK it won't open a PR if there's a conflict... one can set ignore_conflicts https://docs.mergify.com/actions/backport/#options in which case, it'll just open the PR with conflicts and a maintainer needs to manually resolve it (either in GitHub UI or by directly working on the branch the bot created)