Update files to use pajamas button

What does this MR do and why?

Updates buttons in projects/blob/_editor, projects/blob/_header, projects/merge_requests/_nav_btns, views/shared/issuable, and projects/project_templates/_project_fields_form to use Pajamas::ButtonComponent.

Resolves #363897 (closed), #363901 (closed), #363899 (closed), #363894 (closed), #363904 (closed).

Screenshots or screen recordings

	Before	After
projects/blob/_editor
projects/blob/_header	cell
projects/merge_requests/_nav_btns and shared/issuable/_feed_buttons
projects/project_templates/_project_fields_form
views/shared/blob/_markdown_buttons

How to set up and validate locally

projects/blob/_editor

• Navigate to project/-/new/master/ to create a new file
• Verify that the wrap button toggles and changes the wrapping behavior

projects/blob/_header

• Adjust this if/else so that it always renders the file-holder class
• Navigate to an existing file in a project
• Verify that the Fork/Cancel buttons show

projects/merge_requests/_nav_btns and shared/issuable/_feed_buttons

• Go to the issues page for a project (/-/issues)
• Verify that the Edit Issues and New Issue buttons are present and function
• Verify that the subscribe to RSS feed and Subscribe to Calendar buttons are present and work as expected

projects/project_templates/_project_fields_form

• Go to create a new project
• Select "Create from template" (/projects/new#create_from_template)
• Verify that the "Change template" button works as expected

views/shared/blob/_markdown_buttons

• Go to an issue and scroll down to the comment box
• The attach file and expand to full screen buttons should be visible and work as expected

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.