Make stricter LIMIT param checking for -1, ''

Prerequesites:

• Moodle needs to be workign to send email, I suggest using $CFG->divertallemailsto so we don't have to worry about email addressses.
• My config:

  $CFG->smtphosts = 'smtp.highway1.com.au';

  $CFG->divertallemailsto = 'talktodan@gmail.com';

Testing:

1. Run behat & phpunit tests and ensure there are no debugging messages about invalid params
2. Browse around Moodle with DEBUG_DEVELOPER turned on and VERIFY there are not invalid params debugging messages when doing the following:
3. Ensure that the teacher role is selected in 'Course contacts' admin setting (Site administration / ▶ Appearance / ▶ Courses '')
4. In a course, enable the self enrolment activity.
5. As a student who isn't enrolled, go to the course - they should be asked to self enrol
6. Click 'enrol me'
7. You should recieve an email welcome message to the course..
8. In a course with activity...
9. Visit reports -> logs page and click 'get logs'
10. Change the option to download the files as csv, ODS and excel format and ensure each file downlaods and opens correctly.
11. Change the 'all particpants' dropdown to select a specific user and click 'get logs'
12. As admin: Add 'paella' to your interests in your user profile
13. Click on 'paella' in your user profile page. Ensure you see yourself in the list of users tagged with 'paella'.