Add tab width option to user preferences

What does this MR do?

This MR implements an MVP for #13904 (closed).

Tab width option is added to user preferences. The specified tab width is applied as a tab-size property to the body element, which is inherited by:

• inline and parallel diffs in commits and MRs
• code spans and code blocks in rendered markup (Markdown, Rdoc, etc.)
• blame
• merge conflicts
• code suggestions
• snippets
• blob previews
• Sherlock file samples
• CI logs
• basically any pre element or elements with white-space: pre-wrap or white-space: break-spaces.

This setting doesn't apply to Web IDE, file editor or markdown editor. These cases can be addressed in future iterations.

Also in the issue some people asked for project-wide tab width settings based on .editorconfig, this is not covered by this MVP too.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers