Send email reminders for users to fill their tasks in

[jaragunde]

We could have a service to send an email to users who haven't filled their tasks in for a certain amount of time, for example one week. The amount of time and the contents of the message should be configurable.

[jaragunde]

Let's define this task a bit more:

• Triggers are expressed as "number of days since the last saved task by the user".
• Warnings will be emailed to users and, optionally, to a list of CC recipients.
• The contents of the email will be customizable.
• There will be two triggers to send emails with different degrees of severity.
• Every trigger level will be configurable independently; will have a specific text and list of recipients.

[jaragunde]

Also, a side effect of this task is that we need to add an "email" field to User entities.

[jaragunde]

Reviewed at PR #400:

commit 25e95671af37cf1a3a7ff97210833befce161c23 
Merge: 5c4e79a1 63670af7
Author: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Date:   Tue Jun 5 16:28:33 2018 +0200

    [#334] Creation of a email fill in reminder web services
    
    * Service to send reminders to users who haven't filled their tasks
      for a while.
    * Service for managers to check who's not filling PhpReport lately.
    
    (Merge pull request #400 from Igalia/no-fill-email-alert)

commit 63670af766b506c5c308c094902e21b6678432cd 
Author: Pablo Abelenda <pabelenda@igalia.com>
Date:   Tue May 22 15:54:06 2018 +0200

    [#334] Several changes over the no-fill alert service
    
     * Splitted the summary and individual emails in two jobs
     * Summary is meant to include all the people beyond boundaries
     * Added new template file to craft the managers summary
     * Excluded first warned people from the managers summary
     * Individual alerts are meant to be sent just one time each
     * Added an easy way to exlude users from the checks/report

commit 976f1b114c49c8625bee5fb4656c4bcac4dcc5ee
Author: Pablo Abelenda <pabelenda@igalia.com>
Date:   Tue May 8 08:53:06 2018 +0200

    [#334] Initial version of the Email reminder service
    
     * Added configuration parameters
     * Added a couple of email templates
     * Initial version of the web service