add reporting for blocked issues, pull requests

This adds a listing of issues with a `blocked` label and a listing of open pull requests. Both I think are worth knowing about and working hard to minimize. Feedback welcome, here is some sample output from just now:

```
# Github activity

## Blocked Issues
  - **api** [Bad etags on list endpoints](https://github.com/heroku/api/issues/989) by jclem assigned to geemus
  - **api** [Document slug format!](https://github.com/heroku/api/issues/898) by pedro assigned to friism
  - **core** [Sunset production flag](https://github.com/heroku/core/issues/1780) by pedro assigned to mfine

## Pull Requests
  - **api** [Fix formations by type responses](https://github.com/heroku/api/pull/1047) by brandur assigned to null
  - **api** [silent signup for manager-api](https://github.com/heroku/api/pull/1044) by sclasen assigned to null
  - **api** [Retire production mode](https://github.com/heroku/api/pull/1034) by pedro assigned to pedro
  - **api** [Stop shortening app/release git_hash?](https://github.com/heroku/api/pull/1025) by pedro assigned to null
  - **api** [Generate docs off schema.json](https://github.com/heroku/api/pull/1004) by pedro assigned to geemus
  - **api** [Allow undocumented path versioning](https://github.com/heroku/api/pull/1023) by brandur assigned to null
  - **api** [Additional HTML email templates](https://github.com/heroku/api/pull/1020) by dominic assigned to null
  - **api** [syncup v3 account to schema updates](https://github.com/heroku/api/pull/1014) by sclasen assigned to null
  - **api** [Start raising errors on endpoints that fail behavior checks](https://github.com/heroku/api/pull/1009) by brandur assigned to null
  - **api** [Default range unit](https://github.com/heroku/api/pull/988) by geemus assigned to geemus
  - **api** [Generalized V3 find operation](https://github.com/heroku/api/pull/987) by brandur assigned to null
  - **api** [Managing collaborators now requires `write-protected` scope](https://github.com/heroku/api/pull/950) by brandur assigned to null
  - **api** [Nuke RHs, step 1](https://github.com/heroku/api/pull/917) by pedro assigned to null
  - **api** [Nuke RHs, step 0](https://github.com/heroku/api/pull/915) by pedro assigned to null
  - **api** [Add cache_size field to apps.](https://github.com/heroku/api/pull/893) by technomancy assigned to null
  - **api** [Drop `shens` table + unneeded index](https://github.com/heroku/api/pull/773) by brandur assigned to brandur
  - **api** [Remove legacy invoices table](https://github.com/heroku/api/pull/771) by brandur assigned to brandur
  - **api** [UTC step 1](https://github.com/heroku/api/pull/687) by pedro assigned to null
  - **api** [Prepare for event stream v1](https://github.com/heroku/api/pull/578) by brandur assigned to null

## Brandur
  - **api** opened [pull Fix formations by type responses](https://github.com/heroku/api/pull/1047)
  - **api** opened [issue V3 dynos API restart/stop returns empty strings](https://github.com/heroku/api/issues/1038)
  - **api** closed [issue V3 dynos API restart/stop returns empty strings](https://github.com/heroku/api/issues/1038)
  - **api** pushed [Remove "payments" access on the delinquency endpoints](https://github.com/heroku/api/commits/fe07e2bf13a9b9945d957163ba9a7bfeccb252f5)
  - **api** pushed 2 commits: [Merge branch 'master' of github:heroku/api](https://github.com/heroku/api/commits/d6029fe605cc21f93a51c5d3f40a73297efc65ae)

## Wesley Beary
  - **api** closed [pull Scott moar schema: Bunch-o-fixes](https://github.com/heroku/api/pull/1041)
  - **api** reopened [issue 304s should not count against the limit?](https://github.com/heroku/api/issues/757)
  - **api** reopened [issue Strip linebreaks from errors?](https://github.com/heroku/api/issues/877)
  - **core** closed [issue V3 Heroku labs route consistency fixes](https://github.com/heroku/core/issues/1272)
  - **api** pushed 8 commits: [Merge pull request #1041 from heroku/scott-moar-schema](https://github.com/heroku/api/commits/1d0d7bc1b4d4f7673c443c6a5153756c56e3dd56)

## Mark Fine
  - no tracked activity

## Pedro Belo
  - **api** closed [pull Clean up max dynos validation](https://github.com/heroku/api/pull/1019)
  - **api** opened [issue Investigate statement timeout + DWH](https://github.com/heroku/api/issues/1037)
  - **api** opened [issue Alert on database feedback](https://github.com/heroku/api/issues/1039)
  - **api** closed [issue Platform H99s / Maestro jobs contention](https://github.com/heroku/api/issues/904)
  - **api** opened [issue Don't change default SSL setting in Excon](https://github.com/heroku/api/issues/1043)

# On-call Schedule
  - API Primary: Brandur
  - API Secondary: Pedro Belo

# On-call alerts
  - No incidents

```