[snap] Remove personal-files interface

Use of the `personal-files` interface is reserved for vetted publishers.

The interface requires a validation, but we need to publish at least
one package first. So let's skip that part, release a version and ask
for the interface access in a second time.

Also add a workflow to build & test the snap package.

.github/workflows/packaging-linux-snap.yml

@@ -0,0 +1,25 @@
+name: Linux snap
+
+on:
+  pull_request:
+    paths:
+      - .github/workflows/packaging-linux-snap.yml
+      - snapcraft.yaml
+
+jobs:
+  snap:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build
+        uses: snapcore/action-build@v1
+        id: snapcraft
+      - name: Install
+        run: sudo snap install --dangerous ${{ steps.snapcraft.outputs.snap }}
+      - name: Test
+        run: |
+          httpie.http --version
+          httpie.https --version
+          # Auto-aliases cannot be tested when installing a snap outside the store.
+          # http --version
+          # https --version