Add event placeholder support to more fields (#6098)

* Add event placeholder support to more fields

I found it useful to be able to use the `{event}` placeholder in
some fields such as the "End of presale text" because I don't need
to fix the texts creating a new event by cloning another event.
I made placeholders available to the other fields as well (where not too
difficult). Specifically `presale_has_ended_text`, `voucher_explanation_text`
`banner_text`, `banner_text_bottom` and `event_info_text`.
In addition, I grouped them under a new `texts` variable in the `context`
(including `frontpage_text` which was part of the root `context` previously).

* change compute location

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Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/control/forms/event.py

@@ -671,6 +671,11 @@ class EventSettingsForm(EventSettingsValidationMixin, FormPlaceholderMixin, Sett
 
     base_context = {
         'frontpage_text': ['event'],
+        'presale_has_ended_text': ['event'],
+        'voucher_explanation_text': ['event'],
+        'banner_text': ['event'],
+        'banner_text_bottom': ['event'],
+        'event_info_text': ['event'],
     }
 
     def _resolve_virtual_keys_input(self, data, prefix=''):