Add generic PMID and DOI functions for HTML text

[RudolfCardinal]

Checklist for new tasks:

• [ ] Add task to relevant menus
• [ ] Generate code documentation
• [ ] Update docs/source/changelog.rst
• [ ] Test documentation rebuilds without errors
• [ ] If the text of the task cannot be submitted here for copyright reasons, there is a template XML file under server/camcops_server/extra_string_templates.

Please [X] all the boxes above that apply