Get thumbnail from app (#68)

* Renamed multipart filed name 'files' to 'assetData'. 
* Added an additional field name of 'thumbnailData' to multipart form.
* Implemented upload mechanism for thumbnail directly from the mobile client.
* Removed dead code
* Implemented a version checking mechanism.

server/src/constants/server_version.constant.ts

@@ -0,0 +1,9 @@
+// major.minor.patch+build
+// check mobile/pubspec.yml for current release version
+
+export const serverVersion = {
+  major: 1,
+  minor: 3,
+  patch: 0,
+  build: 0,
+};