Fix "Resolve conflicts" button not appearing for some users

Closes #47954

See merge request gitlab-org/gitlab-ce!29535

CE backport for gitlab-org/gitlab-ee!12309: Remove merge request pipeline check in mergeable method

See merge request gitlab-org/gitlab-ce!29074

Removes the "target branch advanced" warning from the frontend
as it is no longer needed due to the new merge trains feature.

Previously the frontend assumed that the user had to be able to merge to
that project in order to resolve conflicts. However, this is overly
restrictive, as the user only has to be able to push to the source
branch. In fact, appending the text /conflicts to the merge request
would bring up the conflict resolution page.

This confusion happens when a project contains a protected branch that
only allows maintainers to push. Users with Developer access no longer
have permission to merge, but they still can create branches in that
project.

To fix this issue, we now loosen the permission check for the "Resolve
conflicts" button and only check for access to push to the source
branch. This is consistent with what the backend does in
MergeRequests::Conflicts::ListService#can_be_resolved_by?.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/47954

This commit updates the merge request widget's "Merge" button to
support merge trains.

A few minor frontend changes to complete the refactoring
from MWPS to the more generic autoMergeStrategy.

We have one auto merge strategy today - Merge When Pipeline
Succeeds.

In order to add more strategies for Merge Train feature,
we abstract the architecture to be more extensible.

Removed arguments

Fix spec

* Add Pipeline IID to MR's
* Add Pipeline IID to commit box
* Add Pipeline IID to builds page
* Add Pipeline IID to job sidebar
* Add Pipeline IID to header on view pipeline pages
* Add changelog entry
* Update tests

[CE Port] Rename vue_component_helper to text_helper

See merge request gitlab-org/gitlab-ce!28358