restructure into "server" and "ui" subdirs

Besides being more clear about what belongs to which, this helps with
docker caching. The server and ui parts are only rebuilt when their
respective subdirectories change.

Extend this a bit further by making the webpack build not depend on
the target architecture. And adding cache dirs so parts of the server
and ui build process can be reused when layer-wide caching fails.

.github/workflows/ci.yml

@@ -27,7 +27,7 @@ jobs:
         path: |
           ~/.cargo/registry
           ~/.cargo/git
-          target
+          server/target
         key: ${{ matrix.rust }}-cargo-${{ hashFiles('**/Cargo.lock') }}
     - name: Install dependencies
       run: sudo apt-get update && sudo apt-get install libavcodec-dev libavformat-dev libavutil-dev libncurses-dev libsqlite3-dev pkgconf
@@ -38,7 +38,7 @@ jobs:
         toolchain: ${{ matrix.rust }}
         override: true
     - name: Test
-      run: cargo test ${{ matrix.extra_args }} --all
+      run: cd server && cargo test ${{ matrix.extra_args }} --all
   js:
     name: Build and lint Javascript frontend
     runs-on: ubuntu-20.04
@@ -57,6 +57,6 @@ jobs:
         key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
         restore-keys: |
           ${{ runner.os }}-yarn-
-    - run: yarn install
-    - run: yarn build
-    - run: yarn lint
+    - run: cd ui && yarn install
+    - run: cd ui && yarn build
+    - run: cd ui && yarn lint