Split attachment paths (#848)

* fixed attachment statistics for sqlite

* Split attachment path into internal and external path, so the external source URL can be retained after a file is downloaded

* Make internal and external path for attachments nullable, to make clear that they have no internal or external path

* Added migrations for nullable columns for postgres and mysql

* Added migration for nullable internal and external pathes for sqlite

* Added translations

* Fixed upload error

* Restrict length of filename badge in attachment edit view

* Improved margins with badges in attachment edit

* Added a link to view external version from attachment edit

* Let media_url  stay in API attachments responses for backward compatibility

---------

Co-authored-by: jona <a@b.c>
Co-authored-by: Jan Böhmer <mail@jan-boehmer.de>

src/Services/EntityMergers/Mergers/EntityMergerHelperTrait.php

@@ -247,7 +247,8 @@ trait EntityMergerHelperTrait
     {
         return $this->mergeCollections($target, $other, 'attachments', fn(Attachment $t, Attachment $o): bool => $t->getName() === $o->getName()
             && $t->getAttachmentType() === $o->getAttachmentType()
-            && $t->getPath() === $o->getPath());
+            && $t->getExternalPath() === $o->getExternalPath()
+            && $t->getInternalPath() === $o->getInternalPath());
     }
 
     /**