Replace the tinder gem by bare HTTP requests

## What does this MR do?

It removes the `tinder` gem (used to talk to the Campfire API) and replaces its use by bare HTTP requests.

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

N/A.

## Why was this MR needed?

To simplify the potential dependency hell discussed in #13333.  
The initial issue was about updating `http_parser.rb` which was required by `twitter-stream` which was required by `tinder`.  
Those 3 gems are not needed anymore.  

## What are the relevant issue numbers?

#13333

## Screenshots (if relevant)

N/A.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
Closes #13333

See merge request !5758