Use Jenkins GitLab Plugin

Fixes #106

Deprecate the existing Jenkins service, which uses GitLab Hook plugin.
Adds support for GitLab Plugin which utilizes the GitLab commit status
API. The overall experience is much better.

See merge request !79

Add weight field to the issue

Fixes #70 
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !76

[ci skip]

GitLab Pages

Yay! It works!

**Key facts:**
- The pages are created when build artifacts for `pages` job are uploaded
- Pages serve the content under: http://group.pages.domain.com/project
- Pages can be used to serve the group page, special project named as host: group.pages.domain.com
- User can provide own 403 and 404 error pages by creating 403.html and 404.html in group page project
- Pages can be explicitly removed from the project by clicking Remove Pages in Project Settings
- The size of pages is limited by Application Setting: max pages size, which limits the maximum size of unpacked archive (default: 100MB)
- The public/ is extracted from artifacts and content is served as static pages
- Pages asynchronous worker use `dd` to limit the unpacked tar size
- Pages needs to be explicitly enabled and domain needs to be specified in gitlab.yml
- Pages are part of backups
- Pages notify the deployment status using Commit Status API
- Pages use a new sidekiq queue: pages
- Pages use a separate nginx config which needs to be explicitly added

**What is missing:**

- The Omnibus changes (I'll do it tomorrow)
- The proper documentation for Pages in this repo (@axil Can you help?)
- The blessing of everyone involved
- @DouweM help with Project Settings page: I put there the GitLab Pages deployed link and remove button 

**Related:**
- Omnibus changes: gitlab-org/omnibus-gitlab!565



See merge request !80

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>

- Move UploadsTransfer to ProjectTransfer and inherit from this to UploadsTransfer and PagesTransfer

Revert "Store pages in shared/pages/fqdn/fqdn/public or shared/pages/fqdn/subpath/public - makes it simpler to implement CNAMEs in future"

This reverts commit 86a2a78f0d13a678899460638add6b862059433e.

This reverts commit 0d73bd93bab349d84910cf14773633b8a66df466.

Store pages in shared/pages/fqdn/fqdn/public or shared/pages/fqdn/subpath/public - makes it simpler to implement CNAMEs in future

- Fix markdown
- Remove how it works section, maybe add it at a later point