Consolidate markdown table styles

The problem

Markdown tables styles are spread across several stylesheets and suffer from complex cascading issues. Part of the table styles are derived from a class definition included in an md-typography mixin:

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/stylesheets/framework/typography.scss#L139

It’s worth noticing that at the same time, the class definition mentioned above, inherits from a mix of our custom table styles (https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/stylesheets/framework/tables.scss#L6 ) and bootstrap’s table-bordered class.

The rest of the of the markdown table customizations are located in a markdown-table mixing: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/stylesheets/framework/mixins.scss#L27

This organization makes table styles brittle and leads to inconsistencies like #49071 (closed) and #57655 (closed).

Risks

Since markdown tables appear extensively across the application, performing a refactor is a high-risk operation. I recommend creating visual regression tests that allow us to prevent further problems while improving the current design.