Add posting_editor_subject_prepend/append template events

Adds two events:

• posting_editor_subject_prepend
• posting_editor_subject_append

These events show up in styles/prosilver/template/posting_editor.html, directly between the <dd> and the <input> tag for the subject field.

These allow you to add things directly before and after the subject input text box on the posting page.

My use case is my https://github.com/imkingdavid/prefixed extension, I'd like to add a javascript drop down menu right before the field so that users can add prefixes to their topic titles.

I will be submitting the PR for this.