Send email notification when queue reaches a certain size

[tsteur]

fixes #71

Could also add translation keys for the email but think not needed as sent from CLI anyway etc.

Also noticed uninstall was missing but it would currently not uninstall any queue list if a MySQL backend is used... need to create a bug for this. (#73)

[tsteur]

cc @mattab

kept it simple but useful!

[diosmosis]

Code looks good, build is failing here: https://travis-ci.org/matomo-org/plugin-QueuedTracking/jobs/366338040 due to DEFAULT_NOTIFY_EMAILS value being an array. Will be able to test locally Sunday.

[tsteur]

Ouch yeah... I'll fix it tomorrow.

[tsteur]

test fixed

[diosmosis]

Couple things I noticed testing:

1. I noticed that the email will contain the sizes for every queue, but will also say that the queue size threshold has been reached. This might suggest to a user that there's a bug if they see that one queue is over the threshold, but the other queues are fine.

To make it clearer, do you think it would be a good idea to differentiate between which queues are ok which aren't. Like having two sections:

This is a notification that the threshold %s for a single queue has been reached.

The following queue sizes are greater than the threshold:
- ...
- ...
- ...

The remaining queue sizes, which are below the threshold, are listed below:
- ...
- ...

2. I think it might be useful for debugging/testing to add some info/debug logs to notifyQueueSize(). Eg, https://github.com/matomo-org/plugin-QueuedTracking/pull/72/files#diff-e643977fa3ad1b1cee4d42603acb0c44R52 & https://github.com/matomo-org/plugin-QueuedTracking/pull/72/files#diff-e643977fa3ad1b1cee4d42603acb0c44R71. Would also put one https://github.com/matomo-org/plugin-QueuedTracking/pull/72/files#diff-e643977fa3ad1b1cee4d42603acb0c44R44 but not sure how really useful that one would be. Might also be good to have a warning in the UI or somewhere else if the threshold is set to 0, though I suppose that wouldn't happen accidentally.

[tsteur]

done