WIP: Make MarkdownField responsible for its own textarea

What does this MR do?

Refactors the MarkdownField Vue component to manage its own <textarea> and behavior.

Before, the <textarea> was managed by the parent component and inserted into the MarkdownField through a slot, and its autosave, autosize, autocomplete, and dropzone behavior was either set up in the parent component, or using GLForm.

This refactor does not (or should not) change any behavior from the perspective of the user, but lays the groundwork that will allow us to add a rich editor to the MarkdownField as an alternative to the <textarea>, in a way that will be transparent to the parent component. See also https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22177.

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/33667

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the style guides