Merge branch 'add_description_to_plain_text' into 'master'

Add issue description to txt/plain emails

## What does this MR do?
Adds issue description to new_issue mail when in text/plain content-type

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

## Why was this MR needed?
Because clients that are rendering emails in text mode can see issue description 

## What are the relevant issue numbers?
#1821 

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5663

CHANGELOG

@@ -67,6 +67,7 @@ v 8.11.0 (unreleased)
   - Sensible state specific default sort order for issues and merge requests !5453 (tomb0y)
   - Fix RequestProfiler::Middleware error when code is reloaded in development
   - Catch what warden might throw when profiling requests to re-throw it
+  - Add description to new_issue email and new_merge_request_email in text/plain content type. !5663 (dixpac)
   - Speed up and reduce memory usage of Commit#repo_changes, Repository#expire_avatar_cache and IrkerWorker
   - Add unfold links for Side-by-Side view. !5415 (Tim Masliuchenko)
 

app/views/notify/new_issue_email.text.erb

@@ -3,3 +3,5 @@ New Issue was created.
 Issue <%= @issue.iid %>: <%= url_for(namespace_project_issue_url(@issue.project.namespace, @issue.project, @issue)) %>
 Author:    <%= @issue.author_name %>
 Assignee:  <%= @issue.assignee_name %>
+
+<%= @issue.description %>

app/views/notify/new_merge_request_email.text.erb

@@ -6,3 +6,5 @@ New Merge Request <%= @merge_request.to_reference %>
 Author:    <%= @merge_request.author_name %>
 Assignee:  <%= @merge_request.assignee_name %>
 
+<%= @merge_request.description %>
+