Add support for client side caching of document page images. The time

the images are cached is controlled by the new setting
DOCUMENTS_PAGE_IMAGE_CACHE_TIME which defaults to 3600 seconds (1 hour).

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/documents/settings.py

@@ -31,6 +31,9 @@ setting_display_height = namespace.add_setting(
 setting_display_width = namespace.add_setting(
     global_name='DOCUMENTS_DISPLAY_WIDTH', default='3600'
 )
+settings_document_page_image_cache_time = namespace.add_setting(
+    global_name='DOCUMENTS_PAGE_IMAGE_CACHE_TIME', default='3600'
+)
 setting_documentimagecache_storage = namespace.add_setting(
     global_name='DOCUMENTS_CACHE_STORAGE_BACKEND',
     default='django.core.files.storage.FileSystemStorage'