- Fixes double escaped text in `mr_widget_rebase.vue`

This commit adds a helper text that describes what will happen when a
user clicks the "Start/Add to merge train when pipeline succeeds"
button.  This text appears at the bottom of the merge request widget.

In addition, this commit renames the "merge_train_info" component to
"merge_train_position_indicator" to avoid ambiguity with the
"merge_train_helper_text" component.

This commit fixes an issue that was causing the merge button dropdown
trigger to be shown even if the dropdown itself was hidden. This commit
also fixes an issue that caused the "Merge Immediately" option not to be
available when merge trains were enabled.

The GitLab UI update changed the spinner selector from `spinner` to
`gl-spinner`, so we need to update it, so that our tests won't fail.

New API endpoint for merge request count
Updates all open tabs at the same time with one call
Restructured API response

API response changed to 401 if no current_user

Added API + JS specs

Fix for Static Check

Updated Count on Open/Close, Assign/Unassign of MR's

Checking if MR Count is refreshed

Added # frozen_string_literal: true to spec

Added Changelog

Extract MR's widget into a separate request

Closes #62723

See merge request gitlab-org/gitlab-ce!29979

This commits extracts /merge_requests/1.json?serializer=widget
Into a separate /merge_requests/1/widget.json endpoint
This will allow to use caching for this request