Add templates components form fields placeholders. Placeholder texts' are as specific as possible based on templates' architecture (so that shared fields have generic placeholder texts).
Normally this shouldn't happen, but due to an incomplete data migration #94
we currently have extensions that have only CMT items and miss the AWR one.
Based on the proposal #93
Email subjects change to the following:
```
Add-on approved: "Genuinely Disgusting"
New comment on Add-on "Genuinely Disgusting"
Add-on rated: "Genuinely Disgusting"
Add-on reported: "Genuinely Disgusting"
Add-on rating reported: "Genuinely Disgusting"
Add-on changes requested: "Genuinely Disgusting"
Add-on review requested: "Genuinely Disgusting"
```
Notification emails can also be viewed in the admin `Email previews` section (with fake objects constructed and passed into email template contexts).
Indicating why the email was sent is not in this change because we currently don't store why **precisely** (can only guess it's either because of `moderators` group or extension follow, the state of either can change after the notification was generated). This can be part of the `Notification` records however, after that the reason can be included in the email.
Reviewed-on: #96
Reviewed-by: Oleg-Komarov <oleg-komarov@noreply.localhost>
