Refine Icon? exclusion in .gitignore

Summary:
Icon? is too general and its causing us to ignore folders like webapp's `topic_icons/icons`. This
diff refines the pattern using a tip from: https://wolfgang.reutz.at/2015/06/16/git-ignore-mac-osx-icon-files/.

Specifically, I removed `Icon?` from the `.gitignore` template and then executed the following:

```
with open('gitignore.template', 'a') as f:
    f.write('Icon\r\r')
    f.write('# Any new entries below this line...')
```

Test Plan:
- Copy the contents of the template into `~/.gitignore`.
- Add a new file to webapp's `topic_icons/icons`.
- `git status`
    - Verify that the new file is tracked by Git.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: mroth

Differential Revision: https://phabricator.khanacademy.org/D30984

gitignore.template

@@ -3,7 +3,10 @@
 *.pyc
 .svn
 Thumbs.db
-Icon?
 *.swp
 tags
 *.class
+Icon
+
+# Any new entries below this line (the above two lines are actually a single Icon\r\r entry).
+#   See: https://wolfgang.reutz.at/2015/06/16/git-ignore-mac-osx-icon-files/.