Let contributors know where to start

Needed becuase: https://gitlab.com/gitlab-org/gitlab-ce/issues/14905#note_12235996

See merge request !4455

Ensure branch cleanup regardless of whether the GitHub import process succeeds

Closes #17766


See merge request !4450

Resolve "SHA parameter in accept merge request API"

Add a `sha` parameter to the MR merge API, which must match the source SHA for the branch to be merged.

Also add the same parameter to the UI:

![MR_SHA](/uploads/616da728695dc19fa7ef7ef6a016ff81/MR_SHA.gif)

@DouweM and I discussed adding some smart feature to that, like updating the source SHA on navigating to the diff tab, but for now it will just require a refresh 😃

Closes #14139.

See merge request !4414

Ensure we don't show TODOS for projects pending delete

## What does this MR do?

Joins the todos on the projects table in order to run the default scope. Also includes a where clause because the default scope is being removed soon.

## Are there points in the code the reviewer needs to double check?

An alternative approach, more like the Issues page, would be to filter down the list by passing user.authorized_projects into the where clause.

Or we could just be more defensive in the view when iterating.

## Why was this MR needed?

Todos page throws 500 error for users with todos in a project pending deletion.

## What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/17813

cc\ @stanhu 

See merge request !4300

Fix deprecation warnings in spec/services/issues/bulk_update_service_spec.rb

Fixes #13313.

See merge request !4441

Remove unused Issuable#is_assigned? method

Fixes #13312.

See merge request !4440

gitlab-org/gitlab-ce/issues/14905#note_12235996

See merge request https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4431

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Add icons and buttons to UI guide

cc @jschatz1 @skyruler 

See merge request !4445

Fix import error handling

Fixes https://gitlab.com/gitlab-com/support-forum/issues/745

This improves import error handling:
- Now if there's an error during importing before the job is scheduled, we also mark the project status as failed.
- Refactored setting the status to failed into one single method.
- Fixed some situations where the error message was missing or simply empty.

See merge request !4366