Fix mobile layout issues in notes form

What does this MR do?

This MR fixes two layout bugs in the notes form:

• The markdown toolbar is broken when the notes form is used in the context of the snippets page.
• The "Toggle thread" button is misaligned.

The MR removes deeply nested CSS in favor of utility classes that target specific elements.

Screenshots or Screencasts (strongly suggested)

Screen	Before	After
Snippets
Toggle thread button
Resolve thread (unchanged)

Snippets notes responsive gif

How to setup and validate locally (strongly suggested)

There are two UIs affected by this change

Snippets

• Activate responsive design mode in your browser of choice. Simulate a small screen device.
• Open an snippet (create one in your local environment if you haven’t yet).
• The notes below the snippet should have a markdown toolbar with horizontally aligned buttons

Merge Requests

• Activate responsive design mode in your browser of choice. Simulate a small screen device.
• Open an MR
• Open a resolvable discussion in the MR.
• The resolve thread buttons should be unchanged.
• The toggle thread button (when the discussion is resolved and the MR page is reloaded) should be aligned with the user’s avatar.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team