Allow account header menu items to be buttons

Problem/Motivation

It sometimes makes more semantic sense to have a button in the account header menu instead of a link. For example when a button should open a modal and doesn't lead to a separate page. However, adding a button element in the account header menu causes an improperly styled menu.

Steps to reproduce

Proposed resolution

Adjust the navbar CSS to style a button the same as a link element. Additionally allow creating buttons in the account header menu by declaring the URL as an empty string (doing this currently leads to no output at all).

Remaining tasks

None

User interface changes

None

API changes

None

Data model changes

None