snap: build snaps on Travis via remote-build.

We invoke builds on Launchpad in stage "deploy" and release it into
the edge channel of the Snap Store.

The deploy stage is blocked on fails of the test stage, so we don't
release snaps with failing tests.

.travis.yml

@@ -25,3 +25,28 @@ jobs:
         on:
           repo: solvespace/solvespace
           tags: true
+    - stage: deploy
+      name: "Snap"
+      os: linux
+      dist: bionic
+      addons:
+        snaps:
+          - name: snapcraft
+            confinement: classic
+      script: ./.travis/build-snap.sh
+      deploy:
+        - provider: snap
+          skip_cleanup: true
+          snap: pkg/snap/*.snap
+          channel: edge
+          on:
+            branch: master
+            tags: false
+        - provider: snap
+          skip_cleanup: true
+          snap: pkg/snap/*.snap
+          channel: edge,beta
+          on:
+            branch: master
+            tags: true
+

.travis/build-snap.sh

@@ -0,0 +1,15 @@
+#!/bin/sh -xe
+
+lp_data_dir="$HOME/.local/share/snapcraft/provider/launchpad"
+lp_credentials="$lp_data_dir/credentials"
+
+mkdir -p "$lp_data_dir"
+openssl aes-256-cbc -K $encrypted_c4bc81f026a2_key -iv $encrypted_c4bc81f026a2_iv \
+  -in .travis/launchpad-credentials.enc \
+  -out "$lp_credentials" -d
+chmod 600 "$lp_credentials"
+
+./pkg/snap/build.sh remote-build \
+  --launchpad-user solvespace \
+  --launchpad-accept-public-upload \
+  --build-on=amd64,arm64,armhf,i386