Improve notifications page (mastodon#12497)

Currently notifications page seems a bit cluttered with no clear
separation between e-mail and filtering settings. This commit tries to
address them by adding clear separation with headers, hints and removing
continuously reused texts for events checkboxes.

app/views/settings/preferences/notifications/show.html.haml

@@ -4,6 +4,10 @@
 = simple_form_for current_user, url: settings_preferences_notifications_path, html: { method: :put } do |f|
   = render 'shared/error_messages', object: current_user
 
+  %h4= t('notifications.email_events')
+
+  %p.hint = t('notifications.email_events_hint')
+
   .fields-group
     = f.simple_fields_for :notification_emails, hash_to_object(current_user.settings.notification_emails) do |ff|
       = ff.input :follow, as: :boolean, wrapper: :with_label
@@ -21,6 +25,8 @@
     = f.simple_fields_for :notification_emails, hash_to_object(current_user.settings.notification_emails) do |ff|
       = ff.input :digest, as: :boolean, wrapper: :with_label
 
+  %h4 = t('notifications.other_settings')
+
   .fields-group
     = f.simple_fields_for :interactions, hash_to_object(current_user.settings.interactions) do |ff|
       = ff.input :must_be_follower, as: :boolean, wrapper: :with_label

config/locales/en.yml

@@ -886,6 +886,10 @@ en:
       body: 'Your status was boosted by %{name}:'
       subject: "%{name} boosted your status"
       title: New boost
+  notifications:
+    email_events: Events for e-mail notifications
+    email_events_hint: 'Select events that you want to receive notifications for:'
+    other_settings: Other notifications settings
   number:
     human:
       decimal_units:

config/locales/simple_form.en.yml

@@ -150,14 +150,14 @@ en:
         text: Why do you want to join?
       notification_emails:
         digest: Send digest e-mails
-        favourite: Send e-mail when someone favourites your status
-        follow: Send e-mail when someone follows you
-        follow_request: Send e-mail when someone requests to follow you
-        mention: Send e-mail when someone mentions you
-        pending_account: Send e-mail when a new account needs review
-        reblog: Send e-mail when someone boosts your status
-        report: Send e-mail when a new report is submitted
-        trending_tag: Send e-mail when an unreviewed hashtag is trending
+        favourite: Someone favourited your status
+        follow: Someone followed you
+        follow_request: Someone requested to follow you
+        mention: Someone mentioned you
+        pending_account: New account needs review
+        reblog: Someone boosted your status
+        report: New report is submitted
+        trending_tag: An unreviewed hashtag is trending
       tag:
         listable: Allow this hashtag to appear in searches and on the profile directory
         name: Hashtag