ZIP Imports: Added validation message display, added testing

Testing covers main UI access, and main non-successfull import actions.
Started planning stored import model.
Extracted some text to language files.

lang/en/entities.php

@@ -45,6 +45,9 @@ return [
     'default_template_select' => 'Select a template page',
     'import' => 'Import',
     'import_validate' => 'Validate Import',
+    'import_desc' => 'Import books, chapters & pages using a portable zip export from the same, or a different, instance. Select a ZIP file to import then press "Validate Import" to proceed. After the file has been uploaded and validated you\'ll be able to configure & confirm the import in the next view.',
+    'import_zip_select' => 'Select ZIP file to upload',
+    'import_zip_validation_errors' => 'Errors were detected while validating the provided ZIP file:',
 
     // Permissions and restrictions
     'permissions' => 'Permissions',

lang/en/errors.php

@@ -105,6 +105,11 @@ return [
     'app_down' => ':appName is down right now',
     'back_soon' => 'It will be back up soon.',
 
+    // Import
+    'import_zip_cant_read' => 'Could not read ZIP file.',
+    'import_zip_cant_decode_data' => 'Could not find and decode ZIP data.json content.',
+    'import_zip_no_data' => 'ZIP file data has no expected book, chapter or page content.',
+
     // API errors
     'api_no_authorization_found' => 'No authorization token found on the request',
     'api_bad_authorization_format' => 'An authorization token was found on the request but the format appeared incorrect',

lang/en/validation.php

@@ -106,7 +106,7 @@ return [
     'uploaded'             => 'The file could not be uploaded. The server may not accept files of this size.',
 
     'zip_file' => 'The :attribute needs to reference a file within the ZIP.',
-    'zip_model_expected' => 'Data object expected but ":type" found',
+    'zip_model_expected' => 'Data object expected but ":type" found.',
 
     // Custom validation lines
     'custom' => [