rustfmt everything

I want to make the project more accessible by not expecting folks to
match my idiosyncratic style. Now almost [1] everything is written
in the "standard" style. CI enforces this.

[1] "Almost": I used #[rustfmt::skip] in a few sections where I felt
aligning things in columns significantly improves readability.

.github/workflows/ci.yml

@@ -17,6 +17,8 @@ jobs:
         include:
           - rust: nightly
             extra_args: "--features nightly --benches"
+          - rust: stable
+            extra_components: rustfmt
     runs-on: ubuntu-20.04
     steps:
     - name: Checkout
@@ -37,8 +39,12 @@ jobs:
         profile: minimal
         toolchain: ${{ matrix.rust }}
         override: true
+        components: ${{ matrix.extra_components }}
     - name: Test
       run: cd server && cargo test ${{ matrix.extra_args }} --all
+    - name: Check formatting
+      if: matrix.rust == 'stable'
+      run: cd server && cargo fmt --all -- --check
   js:
     name: Build and lint Javascript frontend
     runs-on: ubuntu-20.04