FEATURE: allow customizing default timezone for email. (#32964)

1. Changes default email format to llll (eg: Tue, May 8, 2018 2:00 AM)
from 2018-05-08T00:00:00Z UTC which is not nice on the eyes
2. Adds `discourse_local_dates_email_timezone` which allows configuring
default timezone in emails
3. Improved help text on site settings (format / timezone)

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Co-authored-by: Gary Pendergast <gary@pento.net>

plugins/discourse-local-dates/config/locales/server.en.yml

@@ -3,4 +3,5 @@ en:
     discourse_local_dates_enabled: "Enable the discourse-local-dates feature. This will add support to local timezone aware dates in posts using the [date] element"
     discourse_local_dates_default_formats: "Frequently used date time formats, see: <a target='_blank' rel='noopener' href='https://momentjs.com/docs/#/parsing/string-format/'>momentjs string format</a>"
     discourse_local_dates_default_timezones: "Default list of timezones, must be a valid <a target='_blank' rel='noopener' href='https://en.wikipedia.org/wiki/List_of_tz_database_time_zones'>TZ</a>"
-    discourse_local_dates_email_format: "Format used to display a date in emails"
+    discourse_local_dates_email_format: "Format used to display a date in emails, examples: 'LLLL', 'llll' or 'YYYY-MM-DDTHH:mm:ss[Z]'. see: <a target='_blank' rel='noopener' href='https://momentjs.com/docs/#/parsing/string-format/'>momentjs string format</a>"
+    discourse_local_dates_email_timezone: "Timezone used to display dates in emails, must be a valid <a target='_blank' rel='noopener' href='https://en.wikipedia.org/wiki/List_of_tz_database_time_zones'>TZ</a>"

plugins/discourse-local-dates/config/settings.yml

@@ -1,6 +1,10 @@
 plugins:
+  discourse_local_dates_email_timezone:
+    default: "Etc/UTC"
+    client: true
+    area: "localization"
   discourse_local_dates_email_format:
-    default: "YYYY-MM-DDTHH:mm:ss[Z]"
+    default: "llll z"
     client: true
   discourse_local_dates_enabled:
     default: true