Bug report template improvements

It includes the following changes:

1. Remove the `**Describe the bug**` title: since it is a title and not
a HTML comment, it encourages the user to start the bug report content
with `**Describe the bug**`

2. On reproduction steps, suggest using the Solidus sandbox, which helps
in order to have a deterministic way to reproduce the bug

3. Add "Current behavior" section, otherwise it would get lost into the
reproduction steps and "Expected behavior" wouldn't have a counterpart
section

.github/ISSUE_TEMPLATE/bug_report.md

@@ -7,21 +7,25 @@ assignees: ''
 
 ---
 
-**Describe the bug**
-<!-- A clear and concise description of what the bug is. -->
+<!-- Describe your issue here: please provide a clear and concise description of what the bug is. -->
 
 **Solidus Version**:
 <!-- Unsure? Check with `Spree.solidus_version` -->
 
 **To Reproduce**
 <!--
-Steps to reproduce the behavior:
+Steps to reproduce the behavior. Please use the Solidus sandbox generated by
+`bundle exec rake sandbox` as starting point, if possible. More about the
+Solidus sandbox here: https://github.com/solidusio/solidus#sandbox
+
 1. Go to '...'
 2. Click on '....'
 3. Scroll down to '....'
-4. See error
 -->
 
+**Current behavior**
+<!-- A clear and concise description of what happens instead of the expected behavior. -->
+
 **Expected behavior**
 <!-- A clear and concise description of what you expected to happen. -->