[Release][v1.38.0] Offline support

This release adds the option to offline a post feed, giving you the freedom to browse while offline. The use case here is if you know ahead of time you are going to be offline you can download a feed beforehand and read it later.

This release also implements several user requests.

Full changelog

• When opening a link to a post on a different instance, added a banner to explaining the instance mismatch and also have an option to switch instances. Also added a loading spinner when switching instances in this way.
• Added a "Make available offline" button to make a feed available offline.
• Changed bookmark button to reflect the current state. Eg. if the post is already bookmarked then the bookmark icon is filled.