Migrate to manifest v3

[v-ko]

Google announced that manifest v2 will be depricated by 17 Jan 2022. I started adapting the extension to the new standard and most of the changes are trivial (as documented in the migration guide), but the webRequest API will be depricated and where possible replaced by declarativeNetRequest. @wanderor , would you be willing to take a look at that, since you implemented the redirect detecion? I'm not really using the extension anymore and won't be able to test extensively even if I get to migrating to the new API.