Link to broadcast page when courseid == siteid on qm.php (#130)

[djarran]

Description: Resolves #130. This PR aligns the block content and qm.php so that the "Compose message" links to broadcast.php when on the site course (ID = 1).

Testing steps:

1. Install plugin

Test existing block functionality

2. Visit course/view.php?id=1
3. Add the Quickmail block
4. Confirm that the Compose message button links to broadcast.php
5. Create or visit a course
6. Add the Quickmail block
7. Confirm that the Compose message button links to compose.php

Test that qm.php now has same behaviour as block functionality regarding "Compose message" button

8. Visit blocks/quickmail/qm.php?courseid=1
9. Confirm that the Compose message button links to broadcast.php
10. Visit blocks/quickmail/qm.php?courseid={courseid}
11. Confirm that the Compose message button now links to compose.php