Basic links saving

[kirillt]

ARK Shelf must be able to save URLs into a folder (should be able to set default folder in settings).

The format is flexible. It can be JSON containing URL, description and maybe other metadata.

The easiest way to enable previews for stored links is to download image using OGP and store it into ZIP archive together with the JSON. Later, when the library will be ready, the app will be able to generate previews directly into necessary folder. Right now, it is supposed to extract the image from ZIP during indexing and saving it as preview (this is sub-optimal due to duplication, but fine for Proof-of-Concept).

ARK Shelf must be able to add link manually by pressing some button or receive sharing intent with URL. Dialogue should be displayed with default description and folder which are possible to be modified by user. Folder picker might be taken from ARK Retouch or from ARK Navigator (we should consider separating folder picker into re-usable repo).

ARK Navigator should be able to process saved files (see #37).