Update diesel, diesel_migrations, and uuid

This implements the major upgrade of the Diesel crate from version 1 to
version 2. The relevant changes are the following:
- Diesel now requires mutable access to the `Connection` to perform any
  database interaction [0].
- The Diesel derive attributes need to be wrapped by `diesel()` now [1].
- The diesel_migration crate has been completely rewritten and the
  `embed_migrations!()` macro had to be replaced [2].
- The `connection::Connection::transaction` closure now takes one
  argument (the database connection). See [3] vs. [4].
  - I also decided to rename `database_connection` to `conn` in these
    cases to better differentiate the two (we could keep/shadow the
    outer variable for a smaller diff but it seems cleaner that way).

Note: The `Arc<PgConnection> -> Arc<Mutex<PgConnection>>` change is only
intended as a PoC. It is not a good idea as the mutex locking makes some
of the async blocks/code completely pointless. This problem already
existed before though. The documentation sates that the requirement of
mutable connections is expected to be a "straightforward change as the
connection already can execute only one query at a time". So this change
should only make matters worse by moving the locking up and therefore
increasing the time for which locks are held. This should only make the
execution a bit slower (not sure if it would even be noticeable) but it
increases the risk of deadlocks which is the real danger.
To avoid this issue we should use a connection pool instead (e.g., via
r2d2) but there are alternatives like trying to move the locking further
down in the code again or rewriting the database logic.

[0]: https://diesel.rs/guides/migration_guide.html#2-0-0-mutable-connection
[1]: https://diesel.rs/guides/migration_guide.html#2-0-0-derive-attributes
[2]: https://diesel.rs/guides/migration_guide.html#2-0-0-upgrade-migrations
[3]: https://docs.diesel.rs/2.0.x/diesel/connection/trait.Connection.html#method.transaction
[4]: https://docs.diesel.rs/1.4.x/diesel/connection/trait.Connection.html#method.transaction

Cargo.toml

@@ -37,8 +37,8 @@ config         = { version = "0.11", default-features = false, features = [ "tom
 csv            = "1"
 daggy          = { version = "0.8", features = [ "serde" ] }
 dialoguer      = "0.10"
-diesel         = { version = "1", features = ["postgres", "chrono", "uuid", "serde_json"] }
-diesel_migrations = "1"
+diesel         = { version = "2", features = ["postgres", "chrono", "uuid", "serde_json"] }
+diesel_migrations = "2"
 filters        = "0.4"
 futures        = "0.3"
 getset         = "0.1"
@@ -74,7 +74,7 @@ tokio-stream   = "0.1"
 typed-builder  = "0.14"
 unindent       = "0.2"
 url            = { version = "2", features = ["serde"] }
-uuid           = { version = "0.6", features = ["serde", "v4"] }
+uuid           = { version = "1", features = ["serde", "v4"] }
 walkdir        = "2"
 which          = "4"
 xdg            = "2"