All: Resolves #1481: New: Allow downloading attachments on demand or automatically (#1527)

* Allow downloading resources automatically, on demand, or when loading note

* Make needToBeFetched calls to return the right number of resources

* All: Improved handling of resource downloading and decryption

* Desktop: Click on resource to download it (and, optionally, to decrypt it)

* Desktop: Better handling of resource state (not downloaded, downloading, encrypted) in front end

* Renamed setting to sync.resourceDownloadMode

* Download resources when changing setting

* tweaks

* removed duplicate cs

* Better report resource download progress

* Make sure resource cache is properly cleared when needed

* Also handle manual download for non-image resources

* More improvements to logic when downloading and decrypting resources

ReactNativeClient/lib/models/Setting.js

@@ -155,6 +155,14 @@ class Setting extends BaseModel {
 				return SyncTargetRegistry.idAndLabelPlainObject();
 			}},
 
+			'sync.resourceDownloadMode': { value: 'always', type: Setting.TYPE_STRING, section: 'sync', public: true, isEnum: true, appTypes: ['mobile', 'desktop'], label: () => _('Attachment download behaviour'), description: () => _('In "Manual" mode, attachments are downloaded only when you click on them. In "Auto", they are downloaded when you open the note. In "Always", all the attachments are downloaded whether you open the note or not.'), options: () => {
+				return {
+					'always': _('Always'),
+					'manual': _('Manual'),
+					'auto': _('Auto'),
+				};
+			}},
+
 			'sync.2.path': { value: '', type: Setting.TYPE_STRING, section:'sync', show: (settings) => {
 				try {
 					return settings['sync.target'] == SyncTargetRegistry.nameToId('filesystem')