See #890: web UI and email notifications on new reports

See #890 (closed):

• Real-time incrementation of notifications counter in UI for moderators when a new report is submitted
• Email notification to moderators when a new report is submitted (if no moderator is found, then admins are notified)
• MODERATION_EMAIL_NOTIFICATIONS_ENABLED configuration setting to globally disable these email notifications
• Include info about the reported object in email:

  @GinnyMcQueen : Is there a way to include what specifically is being reported in email? The email includes a link to the report and who reported it but not what it is. Is there a way to include a title/link to what's being reported so you don't have to click through to the report to find out?

cc @funkwhale/reviewers-front @funkwhale/reviewers-python

Sample email notification

Email Subject:

[node1.funkwhale.test moderation - Takedown request] New report from user@node1.funkwhale.test

Email Message:

user@node1.funkwhale.test just submitted a report in the "Invalid metadata" category.

Reported object: Track - Who Is There
Open public page: https://node1.funkwhale.test/library/tracks/97758
Open moderation page: https://node1.funkwhale.test/manage/library/tracks/97758

Report content:

This is the report content.

Please do something about it.

• To handle this report, please visit https://node1.funkwhale.test/manage/moderation/reports/c29d7938-6d52-4c81-9e8b-20bec22fd883
• To view all unresolved reports (currently 50), please visit https://node1.funkwhale.test/manage/moderation/reports?q=resolved:no

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Your are receiving this email because you are a moderator for node1.funkwhale.test.