Introduce (blob) cache on the client-side

[pascalwengerter]

Idea

• [ ] Write an ADR
• [ ] Introduce generic cache (for strings etc) client-side
• [ ] Use the generic cache as a blob cache (e.g. for avatar profile pictures, file thumbnails/previews)

Expected behaviour

Prevent unnecessary requests

Actual behaviour

Images don't get cached by the browser cache since the pre-signed URL differs for each occurrance of an img

[dschmidt]

possibly related: https://github.com/owncloud/web/issues/5796