Make shipped apps updatable via appstore

Make shipped apps updatable via the appstore. Apps need an ocsid assigned to them. See example file for how to do so. (will add to doc asap).

You can test by:
- setting up a new ownCloud
- adding the videos app linked below
- test different scenarios
 - default app dir writable
 - default app dir not writable

will remove https://github.com/owncloud/core/pull/8808/files#diff-1225d18e3966cf8859db1978ce38fcaeR508 as soon as there are 2 :+1:s 

please review @DeepDiver1975 @PVince81 @schiesbn 
In the future please only run `find` once and store in a variable. [trivial]
No need to change it now.