Option to specify storage location

Add options to specify where to store the following content:
* Main SQLite DB (with all article content). Currently stored in the internal storage, but can be moved to external storage via an option in settings.
* Cached images. Currently stored in external storage.
* Default download location (for articles downloaded as files).

There are two main reasons to have these options:
* Obviously, some may have limited storage. It seems to be less of an issue with modern devices, but it's usually good to have a choice.
* Privacy issue: storing data in external storage exposes it to other apps with storage access (there's plenty of proprietary apps that refuse to function without the storage access permission). An argument can be made that a user doesn't care about their privacy if they allow storage access to untrusted apps, but I'd prefer wallabag app to store everything in the private storage (internal) by default with an option to change that.