Security and safety improvements for gitlab-workhorse integration

Companion to https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/60

- Use a custom content type when sending data to gitlab-workhorse
- Verify (using JWT and a shared secret on disk) that internal API requests came from gitlab-workhorse

This will allow us to build features in gitlab-workhorse that require
more trust, and protect us against programming mistakes in the future.

This is designed so that no action is required for installations from
source. For omnibus-gitlab we need to add code that manages the shared
secret.

See merge request !5907

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Old deployments of Gitlab might have a big number of old events to be
deleted. Such numbers cause the worker to timeout.
I've limited the amount of rows that should be destroyed at once to
10000, and increased how often pruning shall take place to 4 times a
day.

Since contribution calendar shows only 12 months of activity,
events older than that time are not visible anywhere and can be
safely pruned saving big amount of database storage.

Fixes #21164

Fix Sentry not reporting right program for Sidekiq workers

Moves program tag into the global configuration since this doesn't change and since Sidekiq workers get a unique context for each event.

Closes #21410

See merge request !6004

* upstream/master: (107 commits)
  Fix external issue tracker "Issues" link leading to 404s
  Fix CHANGELOG entries related to 8.11 release.
  Fix changelog
  Reduce contributions calendar data payload
  Add lock_version to merge_requests table
  Add hover color to emoji icon
  Fix wrong Koding link
  Capitalize mentioned issue timeline notes
  Fix groups sort dropdown alignment
  Use icon helper
  Fix inline emoji text alignment
  Adds response mime type to transaction metric action when it's not HTML
  Moved two 8.11 changelog entries to 8.12
  Fix markdown link in doc_styleguide.md
  Display project icon from default branch
  Reduce number of database queries on builds tab
  Update CHANGELOG
  Update Issue board documentation
  Label list shows all issues (opened or closed) with that label
  Update CHANGELOG
  ...

Moves program tag into the global configuration since this doesn't
change and since Sidekiq workers get a unique context for each event.

Closes #21410

* upstream/master: (192 commits)
  Added CHANGELOG
  Added unfold test to parallel and added 'diff discussion' context
  Fix Spinach branches spec
  Better first match on this MR also
  Change merge_error column from string to text type
  Fix typo in gitlab_flow.md
  entities: make Environment inherit EnvironmentBasic
  Updated to optimized specs from !5864
  Added 'with an unfolded line should not allow commenting' scenario (line 125)
  Added addtional 'renderable' validator to check 'data-note-type' attr exists
  Allow passing an index to selectRowAtIndex
  Fixed enter key in search input not working
  Fix file links on project page Files view
  Fix incorrect "stopped impersonation" log message
  8.11 is released, long live 8.12
  Also check if Akismet is enabled, before showing the `Submit as spam` button.
  fix location of markdown help location
  Fix for update 8.10-to-8.11.md doc.
  Appease the linter.
  Add Ruby 2.3 upgrade notes.
  ...