fix for usage of baseURI which doesn't work in IE11

[antony]

document.baseURI doesn't exist in IE and causes errors. This attempts to get it and then falls back to looking at the base tag if present.

Fixes #1561 and #1034 as well as an issue where prefetch would also fail in IE as it also depends on baseURI.

Opening in draft because:

• I've not tested it, I will do tomorrow.
• I haven't written any tests for the helper(s)

Before submitting the PR, please make sure you do the following

• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[benmccann]

lgtm

[antony]

Added unit tests (this is the first time I've used typescript, so feel free to suggest a refactor)