Implement sync

[jsmnbom]

We need to implement some kind of options and cache sync between instances of the extension. Many users read on AO3 on both their phone and their pc etc. This means that due to how e.g. the kudos checking works, if on my pc i look at a fic and then after give it kudos on my phone, it will never show as kudosed on the pc.

Since i can easily imagine the cache take more than 1MB (and definitely more than 8,192 bytes per item) then we can't really use the built in browser sync. This means we need to built a custom sync system via e.g. google drive, onedrive, dropbox or similar.