Display and send warnings also when 80 and 90% of the e-mail quota is reached (OSOE-693)

[Piedone]

Currently, administrators only get notified about running out of their e-mail quota if they already did. So, let's add a feature to display a warning on the admin and send out an e-mail (both just as for the current warning) also when 80% of the quota is reached, and when 90%, just with slightly different wording. Also, this warning can be displayed on the admin as a yellow-background warning, vs the quota exhaustion one with a red error background.

Jira issue

[wAsnk]

Are these texts okay?

Admin dashboard warning:

It seems that your site sent out 80% of e-mails from the available quota for this month. E-mail sending will be stopped until next month (any feature sending out e-mails will fail), when the quota is exceeded.

E-mail:
Subject:

[Warning] Your DotNest site has used {0}% of its e-mail quota

Hi,

It seems that your {sitename} site sent out 80% of the available e-mail quota for this month. E-mail sending will be stopped until next month (any feature sending out e-mails will fail), when the quota is exceeded.

Thank you,
Your Admin Crew

[Piedone]

Yep!