Add an issue template for Vulnerability Disclosure

.gitlab/issue_templates/.gitlab/issue_templates/Vulnerability Disclosure.md

+<!---
+Please read this!
+
+This template is for reporting a security vulnerability about GitLab or 
+GitLab.com
+
+Strongly consider reporting via https://hackerone.com/gitlab, as 
+HackerOne is our preferred disclosure platform.
+
+See also:
+
+- https://about.gitlab.com/security/disclosure/
+- https://about.gitlab.com/handbook/engineering/security/#creating-new-security-issues
+- https://about.gitlab.com/handbook/engineering/security/#engaging-the-security-on-call
+
+--->
+
+### Summary
+
+<!-- Summarize the bug encountered concisely. -->
+
+### Steps to reproduce
+
+<!-- Describe how one can reproduce the issue - this is very important. Please use an ordered list. -->
+
+### Example Project
+
+<!-- If possible, please create an example project here on GitLab.com that exhibits the problematic 
+behavior, and link to it here in the bug report. If you are using an older version of GitLab, this 
+will also determine whether the bug is fixed in a more recent version. -->
+
+### What is the current *bug* behavior?
+
+<!-- Describe what actually happens. -->
+
+### What is the expected *correct* behavior?
+
+<!-- Describe what you should see instead. -->
+
+### Relevant logs and/or screenshots
+
+<!-- Paste any relevant logs - please use code blocks (```) to format console output, logs, and code
+ as it's tough to read otherwise. -->
+
+### Output of checks
+
+<!-- If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com -->
+
+#### Results of GitLab environment info
+
+<!--  Input any relevant GitLab environment information if needed. -->
+
+<details>
+<summary>Expand for output related to GitLab environment info</summary>
+
+<pre>
+
+(For installations with omnibus-gitlab package run and paste the output of:
+`sudo gitlab-rake gitlab:env:info`)
+
+(For installations from source run and paste the output of:
+`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)
+
+</pre>
+</details>
+
+#### Results of GitLab application Check
+
+<!--  Input any relevant GitLab application check information if needed. -->
+
+<details>
+<summary>Expand for output related to the GitLab application check</summary>
+<pre>
+
+(For installations with omnibus-gitlab package run and paste the output of:
+`sudo gitlab-rake gitlab:check SANITIZE=true`)
+
+(For installations from source run and paste the output of:
+`sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)
+
+(we will only investigate if the tests are passing)
+
+</pre>
+</details>
+
+### Possible fixes
+
+<!-- If you can, link to the line of code that might be responsible for the problem. -->
+
+
+---
+
+<!-- Do not edit past here unless you are certain of the impact -->
+
+cc @gitlab-com/gl-security/appsec
+
+/label ~"type::bug" ~"bug::vulnerability"
+/confidential