Adds web app manifest file

[roblarsen]

closes #1629

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• [x ] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x ] My code follows the code style of this project.
• [x ] My change requires a change to the documentation.
• I have updated the documentation accordingly.
• [x ] I have read the CONTRIBUTING document.
• [x ] I have added tests to cover my changes.
• [x ] All new and existing tests passed.

This is a clean update of the original PR. The changes were easy enough and I was eyeing that other PR warily.

I'll add documentation this week as part of a documentation pass (unless someone else wants to add some documentation- I will merge the heck out of those PRs)

[coliff]

oops. realised we don't have any documentation about the webapp manifest. I'll get something together and make a PR.

[roblarsen]

It's in the HTML docs, but is very basic.

[coliff]

ah yes, sorry. That looks fine actually...