snap: Start snap support.

- Create a snapcraft.yaml file in snap directory.
  Couldn't put this in packaging directory.
- Use make-snapcraft.py to generate the snapcraft file.
- Use make-snap to make the snap from the snapcraft file.
- Eris is confined to only access projects in home.
- os.setpriority doesn't work in snaps.
- eris-webserver is run as "eris.webserver".

snap/snapcraft.yaml

@@ -0,0 +1,101 @@
+name: eris
+base: core20
+version: 2021.11.29
+summary: Eris maintains an up-to-date set of reports for every file in a codebase.
+description: |
+  Eris maintains an up-to-date set of reports for every file in a codebase.
+
+  A status indicator summarises the state of each report, and a report is viewed
+  by selecting this status indicator with the cursor.
+
+  The reports are cached in the codebase's root directory in a ".eris"
+  directory.
+
+grade: devel
+confinement: strict
+
+apps:
+  eris:
+    command: bin/eris
+    environment:
+      PYTHONPATH: $SNAP/lib/python3.9/site-packages:$SNAP/lib/python3.9/dist-packages:$PYTHONPATH
+    plugs:
+      - home
+  webserver:
+    command: bin/eris-webserver
+    environment:
+      PYTHONPATH: $SNAP/lib/python3.9/site-packages:$SNAP/lib/python3.9/dist-packages:$PYTHONPATH
+    plugs:
+      - home
+      - network-bind
+
+passthrough:
+  layout:
+    $SNAP/usr/bin/php:
+      symlink: $SNAP/usr/bin/php7.4
+
+parts:
+  eris:
+    plugin: python
+    source: .
+    python-packages:
+      - ./termstr
+      - ./lscolors
+      - ./fill3
+      - ./eris
+    requirements:
+      - ./termstr/requirements.txt
+      - ./fill3/requirements.txt
+      - ./eris/requirements.txt
+    build-environment:
+      - SNAPCRAFT_PYTHON_INTERPRETER: python3.9
+    stage-packages:
+      - python3.9-venv
+      - python3-distutils
+      - python3-pkg-resources
+      - binutils
+      - coreutils
+      - cppcheck
+      - dpkg
+      - elinks
+      - file
+      - g++
+      - gcc
+      - genisoimage
+      - git
+      - golang-go
+      - html2text
+      - lua-check
+      - lua5.3
+      - mediainfo
+      - nodejs
+      - p7zip
+      - pandoc
+      - perl
+      - perl-doc
+      - php-cli
+      - pylint
+      - python3-bandit
+      - python3-cairosvg
+      - python3-coverage
+      - python3-mccabe
+      - python3-mypy
+      - python3-pdfminer
+      - python3-pillow
+      - python3-pycodestyle
+      - python3-pydocstyle
+      - python3-pyflakes
+      - python3-pygments
+      - python3-pytest
+      - python3-pytest-cov
+      - rakudo
+      - rpm
+      - ruby2.7
+      - shellcheck
+      - tar
+      - tidy
+      - unrar
+      - unzip
+      - wabt
+      - xz-utils
+      - yamllint