Navigation routes

[olorin99]

Routes now start with the current instance. Community and user routes now use the name rather than numerical id.

So common routes should now be in the following format
https://example.tld/${instance}/u/${user_name}
https://example.tld/${instance}/c/${community_name}/thread/${postId}
https://example.tld/${instance}/c/${community_name}/microblog/${postId}

Allow setting default instance/account using environment variables.

• defaultInstance for the hostname of the instance. E.g. kbin.earth, piefed.social, lemmy.world.
• defaultSoftware for the software type. E.g. mbin, piefed or lemmy.

Environment variables don't seem to affect the web build. Instead add --dart-define $VAR_NAME=$VALUE to the end of flutter build web. E.g. flutter build web --dart-define defaultInstance=piefed.social --dart-define defaultSoftware=piefed should build with piefed.social as the default instance rather than kbin.earth.

[jwr1]

I'd like to create a Docker image for the web frontend once it's ready, so I'll figure out a way we can get environment variables to work with it when the time comes.