Allow custom / shortened url

[brianmackenna]

In written Bennett Institute documents I want to share urls for onwards sharing

Currently this is not possible as urls are very very long https://reports.opensafely.org/reports/antivirals-and-nmabs-for-non-hospitalised-covid-19-patients-coverage-report/ so do not sit well in written text documents.

I know I can handle this with hyperlinks but when we write documents sometimes we don't have control over final published version i.e. urls might be dropped, so it is preferable to insert the full (but snappy) url into the documents

(also an issue for the main website ebmdatalab/thedatalab#48)