Add merge train helper text to merge request widget

What does this MR do?

Adds this helper text section to the merge request widget when the merge request has been added to the project's merge train:

Mockup updates

After some discussion, the text circled above will instead read:

This merge request will [be added to the][start a] merge train when pipeline #12345 succeeds. More information

Refactoring

This merge request also renames the merge_train_info component to merge_train_position_indicator to avoid ambiguity with the new component added in this merge request, merge_train_helper_text.

This MR has a CE backport

gitlab-org/gitlab-ce!30686

Closes #11982 (closed)