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

See merge request !4436

Add tooltips to todo target links



See merge request !4370

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

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

This will be useful when you want to ask for the 
number of items and later iterate over them, without 
needing to ask if the association is load or not.

So you avoid extra database queries

Use the project finder in the todos finder to limit todos to just ones within projects you have access to.

By joining the Todos on the project table.

[ci skip]