Add feature to embed Lemmy for production use

[Nutomic]

This change adds a new feature flag which compiles the Lemmy backend directly into the Lemmybb binary. It can make deployment easier as there is one less thing to build and deploy. It can also make development easier as you dont need to clone and run Lemmy separately.

To try it, execute cargo run --features embed-lemmy. Also works with cargo build --features embed-lemmy.

I also tried to update the installation instructions to take advantage of this. But i dont have any server to test this on, so it would be better if someone else could do it.

[Tealk]

I get the error

cargo run --features embed-lemmy
    Updating git repository `https://github.com/LemmyNet/lemmy.git`
    Updating git submodule `https://github.com/LemmyNet/lemmy-translations.git`
    Updating crates.io index
error: none of the selected packages contains these features: embed-lemmy

[Nutomic]

Are you on the branch embed-lemmy-prod?

[Tealk]

oh i overlooked that, this is what i get output now

[2022-12-22T17:16:21Z INFO  rocket::launch] 🚀 Rocket has launched from http://127.0.0.1:1244
thread 'main' panicked at 'Error connecting to postgres://lemmy:password@localhost:5432/lemmy', /home/tealk/.cargo/git/checkouts/lemmy-2d52b07039afadd8/9f52d4a/crates/db_schema/src/utils.rs:156:56
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[Nutomic]

Yes you need to have postgres installed and configured. I added the instructions to the readme as part of this pr (same instructions as for lemmy development).