Add a license check on multiple issue boards

Closes #2567

See merge request !2317

Add new doc to Burndown Charts + use cases

See merge request !2364

Overview and use cases for Service Desk

See merge request !2337

Add overview and use cases for MR-related docs

See merge request !2350

Only show LDAP sync in progress flash banner on first login

See merge request !2322

We only overwrite `can_create_board?` so it allows creating multiple
boards when available.

And show a mention to the users so they know their boards didn't
disappear.

The `app/controllers/projects/boards_controller.rb` is identical to
the CE version by doing this.

I've split up the `spec/controllers/projects/boards_spec.rb` into the
logic that is on CE en what is on EE. However, they have diverged. So
they are likely (however less than before) to cause conflicts in the
future.

We'd need to move the EE specific spec logic for
`Projects::BoardsController#index` & `Projects::BoardsController#show`
into `spec/controllers/ee/projects/boards_controller.rb` however, some
of the tests in EE have been improved, so those improvements would
need to be ported to CE.

CE upstream: Thursday

See merge request !2361

Add optional sha parameter when approving an MR through the API

Closes #2394

See merge request !2370

Fix creation of push rules via POST API

Closes #2820

See merge request !2367

Split docs for canary deployments and add use cases

See merge request !2359

Refactor push rules docs

See merge request !2362

Add use cases for Deploy Boards

See merge request !2357

All artifacts are now browsable

Closes #2619

See merge request !2356

This works exactly like the same param when merging an MR.

We don't need this in the UI, because you already get a warning in the widget
when the source branch HEAD changes while you're on the page.

If the `max_file_size` parameter were not specified, Grape would
default that value to `nil`, which prevents the model from being
validated and saved. As a result, users would see a JSON response
that had an `id` of `nil`, and the push rule was never created.

Closes #2820

[EE] Fix some N+1 queries in the GET /projects API

See merge request !2363

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

- add use cases for milestones
- delete the content of bd chart [moved to its own doc (last commit)]

drives the attention of the CE user to the existence of burndown charts

- create new doc for bd charts
- add overview and use cases
- add the content from milestones/index.md
- copyedit bd charts existing text

Geo: Added extra SystemCheck checks

Closes #2860 and #2469

See merge request !2354