Add incremental build trace update API

Closes #14537 

Must be merged if we want changes from gitlab-org/gitlab-ci-multi-runner!124 to work.

See merge request !3447

[ci skip]

[ci skip]

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add slack notifications for wiki pages

## What does this MR do?

Lets the Slack service be configured to send notifications when wiki pages are created or edited.

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

I'm just starting to get familiar with the Gitlab codebase and I was unsure on how to get the wiki page url to pass it to the slack message, on whether or not I needed to refactor the create/update methods for wiki pages from the controller to a service (but seemed necessary to test it better), and if I needed to add a column to the web hooks table or if the services table would have been enough. Please let me know if I should change anything and I will improve the MR, thanks for checking :)

## Why was this MR needed?

Related to #563 and fixes #4233.



See merge request !2998

Add raw build trace output

Closes #15308, #15147

Changes in the UI:

- on finished build:

    ![raw-on-finished-build](/uploads/0e0904940db5b381ae064d49343c6508/raw-on-finished-build.png)

- on running build:

    ![raw-on-running-build](/uploads/0e4c800b68c12bdd0cbd2eea732b22ff/raw-on-running-build.png)

See merge request !3767

License templates when creating/editing a LICENSE file

Closes #12804

See merge request !3660

update changelog

Fix javascript errors with invalid json because of commas

fixes #15390 

See merge request !3827

[ci skip]

Define constants only if not defined yet and freeze them

Fixes #15139.

See merge request !3810

Remove XSS vulnerability in Label and Milestone dropdowns

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/15389

See merge request !1951

Fix GitHub project's link in the import page when provider has a custom URL

Closes gitlab-org/gitlab-ee#478

See merge request !3803