Merge snap code into the Certbot repo

* merge .gitignore

* Move snapcraft.yml up one level.

* update source

* move test.sh to tox.ini

* use new tox.ini in .travis.yml

* move snap build code

* make script executable

* remove unused python3-dev

* don't use deprecated classic flag

* go back to stable channel

* add nginx in snap addons

* add deploy steps

* Add comments explaining external tox envs.

* error if not in CI

* don't use --depth

* remove old .travis.yml

* Add big comment about SNAP_TOKEN.

* Set all_branches: true.

* Add repo setting.

* run travis on tags

* Add more documenting comments to .travis.yml.

.gitignore

@@ -51,3 +51,10 @@ tests/letstest/venv3/
 .certbot_test_workspace
 **/assets/pebble*
 **/assets/challtestsrv*
+
+# snap files
+.snapcraft
+parts
+prime
+stage
+*.snap