Remove font family redundancy, fix missed fonts, remove Open Sans

[astorije]

This fixes #138 and #81.

Initially, I was just trying to rebase #377 with master. I noticed a lot of issues with it and ended up completely redoing it.

A few things:

• There should be no visual changes on the default style, except the title + topic in the headers which are a bit more aligned.
• This fixes a few bugs, such as a use of Leto instead of Lato, missed fonts in some of the themes.
• Crypto will require a bit more fixing to be fully non-redundant, but this is for another PR.
• Zenburn and Morning now only use Lato instead of Lato + Open Sans, as these 2 are very similar. Users of these themes will notice a difference, but I'm ready to bet they did not realize their theme was actually using 2 fonts.
• This was discussed and agreed many times, but working on this PR was a massive pain in the butt because of that overriding situation. My next step once this gets merged is to move the font-family declarations of the default theme to example.css (which we should rename to give it a real name, maybe even just Default) and fix the themes accordingly. Looking at existing issues, we already agreed to that, but pinging @thelounge/maintainers here just so they know.

[astorije]

Any takers? I won't have fun fixing potential conflicts on this one :D

[AlMcKinlay]

Yeah, looks fine to me. 👍