Change: Remove mediaSource and v-image-source

We have removed the deprecated `mediaSource` helper function and the `v-image-source` directive.

BREAKING CHANGE for developers: `mediaSource` and `v-image-source` are not available anymore, `loadPreview` should be used once web-pkg is published.

https://github.com/owncloud/web/pull/7935
https://github.com/owncloud/web/issues/7338
https://github.com/owncloud/web/pull/7072
https://github.com/owncloud/web/pull/7350
