Stograde

Same as v2.7.1, but I forgot to bump the version number before tagging v2.7.1.

- 128 - fix a crash from trying to read .cs251tkignore

- 119 / 120 – tweak how the `--web` cli works
- 122 – add `.cs251tkignore` support
- 121 – properly allow missing "optional" files
- 126 – exclude "optional" files from "partial submission" reporting

- Update natsort dependency to v6 (125)

117 – reduces the CI configuration needed to run the toolkit on StoGit to this:

yaml
image: 'stodevx/cs251-toolkit:v2.6.1'
build:
stage: build
script:
- cs251tk --ci


118 – bypasses an issue with compiling certain SD homeworks

- 116 / 115 Fixes an issue with the `webapp` module and the PyPI package
- 113 - Adds a `--ci` flag to be able to report feedback to the student directly in GitLab CI's output

- 112 Fix an issue with SD homeworks and `--web`
- 111 / 109 Fix an issue where asking to record a homework with no spec would panic the toolkit