Remove the help text under the sidebar subscribe button and style it inline

## What does this MR do?

Remove the help text under the sidebar subscribe button and style it inline

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


## Why was this MR needed?
Current help text is over obvious and pushes references too far down. Also the subscribe button is now consistently styled and not a complete block button anymore. approved by @awhildy 

## Screenshots (if relevant)

before:
![image](/uploads/0c9543d2bf184a73b65773caa5f9fc62/image.png)

after:
![image](/uploads/765338823376267427f4a16737e39998/image.png)


## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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 it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #24281

See merge request !7389