Allow custom text on 'almost there' page

Add a new application setting, after_sign_up_text. This is text to be
rendered as Markdown and shown on the 'almost there' page after a user
signs up, but before they've confirmed their account.

Closes #14098.

See merge request !4098

Add a new application setting, after_sign_up_text. This is text to be
rendered as Markdown and shown on the 'almost there' page after a user
signs up, but before they've confirmed their account.

Tweak the styles for that page so that centered lists look reasonable.

This was not a clear cut n+1 query, given that if you're directly subscribed to all issues
that the API is returning you never really need to check for the notes. However, if you're
subscribed to _all_ of them, then for each issuable you need to go once to `notes`, and
once to `users` (for the authors). By preemtively loading notes and authors, at worst you have
1 extra query, and at best you saved 2n extra queries. We also took advantage of this preloading
of notes when counting user notes.

Dismantling Grack::Auth part 1: Git HTTP clients

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/14501

This does not completely get rid of Grack::Auth yet because Git LFS
support is 'behind' it and I would like to not make this MR bigger
than needed.

- changed tests to make HTTP requests instead of calling Rack apps
- added missing test cases for Git HTTP authentication
- moved Git HTTP requests into a 'normal' Rails controller

See merge request !3361

Check if GitHub rate limite API was reached before update Webhooks

## What does this MR do?

Checks if the job needs to sleep, and wait for the rate limit to be reseted before update each Webhook.

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

The import process can fail if the API rate limit was reached during the import process.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/17498

## Screenshots (if relevant)

Not relevant.

See merge request !4509

Conflicts:
	lib/gitlab/workhorse.rb

Add new GitLab CI configuration facade class

## What does this MR do?

This MR is a first iteration of major CI configuration refactoring. See #17139 and #15060 for more details.

## What are the relevant issue numbers?

Closes #17139

See merge request !4462

Ability to prioritize labels

Closes #14189 

See merge request !4009