Part of gitlab-org/gitaly#639

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.

Add a rubocop rule to check if a method 'redirect_to' is used without explicitly set 'status' in 'destroy' actions of controllers

Remove slash commands from Markdown preview and display their outcome next to
the text field.
Introduce new "explanation" block to our slash commands DSL.
Introduce optional "parse_params" block to slash commands DSL that allows to
process a parameter before it is passed to "explanation" or "command" blocks.
Pass path for previewing Markdown as "data" attribute instead of setting
a variable on "window".

See
http://guides.rubyonrails.org/autoloading_and_reloading_constants.html
for more info.
Signed-off-by: NRémy Coutable <remy@rymai.me>

* Add a new DestroyService for Wiki Pages
* Alter WikiPagesController to use the new service

This reverts commit 13e37a3e.

Wiki files (not pages - files in the repo) are just sent to the browser
with whatever content-type the mime_types gem assigns to them based on
their extension. As this is from the same domain as the GitLab
application, this is an XSS vulnerability.

Set a CSP forbidding all sources for scripting, CSS, XHR, etc. on these
files.

- Need to pass in a `page_slug` to the filter, so it can rewrite based
  on the current page (all links are rewritten to the level of the app root).
- The earlier `markdown_preview` endpoint was at the level of the wiki.
  We need to know the current page (for rewriting, as above), so this
  commit moves the endpoint to the level of a wiki page.
- Fix all tests

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

This splits the Markdown rendering and reference extraction phases into
two distinct code bases. The reference extraction phase no longer relies
on the html-pipeline Gem (and any related code) and allows for
extracting of references from multiple HTML nodes in a single pass. This
means that if you want to extract user references from 200 comments you
no longer need to run 200 times N number of queries, instead only a
handful of queries may be needed.

Closes #15527.
Signed-off-by: NRémy Coutable <remy@rymai.me>

update changelog

Closes #2104

Closes #1751

Closes #1592

Closes https://github.com/gitlabhq/gitlabhq/issues/9399

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

write_ was renamed to create_
modify_  was renamed to update_

So now in update action we have next code

def create
  can?(current_user, :create_issue, @issue)
end

def update
  can?(current_user, :update_issue, @issue)
end
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>