Linux-specific fonts: Liberation Sans (#572)

* Added linux-specific fonts

Currently, ubuntu has a lot of issues with fonts.
Both the regular font-family and the data-font-family end up
evaluating to NimbusSans on my system
  (ubuntu's fallback font from helvetica)

NimbusSans unfortunately is noticably too high, cause text in buttons,
emoji on the left pane, and other vertical text alignment to be too high

This diff explicitly says to use Liberation Sans which should
have no effect on windows/mac systems, but should significantly improve
appearance on ubuntu (and hopefully other linuxes)

Both of these fonts are some of the more widely supported linux fonts,
see:
https://www.webfx.com/blog/web-design/a-web-designers-guide-to-linux-fonts/

app/client/ui2018/cssVars.ts

@@ -82,13 +82,13 @@ export const colors = {
 
 export const vars = {
   /* Fonts */
-  fontFamily: new CustomProp('font-family', `-apple-system,BlinkMacSystemFont,Segoe UI,
+  fontFamily: new CustomProp('font-family', `-apple-system,BlinkMacSystemFont,Segoe UI,Liberation Sans,
     Helvetica,Arial,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol`),
 
   // This is more monospace and looks better for data that should often align (e.g. to have 00000
   // take similar space to 11111). This is the main font for user data.
   fontFamilyData: new CustomProp('font-family-data',
-    `Helvetica,Arial,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol`),
+    `Liberation Sans,Helvetica,Arial,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol`),
 
   /* Font sizes */
   xxsmallFontSize:  new CustomProp('xx-font-size',        '8px'),