Add custom links in the side menu

[bofh69]

Is your feature request related to a problem? Please describe.
When using Spoolman it would be really handy to have links in the side menu to my printer's mainsail webpage and other related services.

Describe the solution you'd like
A section in the settings page where one can add/modify/remove extra links which are then shown in the left side menu.

Describe alternatives you've considered
It is possible to change the right component (but I have not even found it), but then upgrades are harder, one can't use the default container and I would have to setup a dev.env. to build the client app.

I can also add bookmarks to the browser, but they're not so user friendly anymore (or they take up screen space).