Load "Open Sans Bold" font-face explicitly

Overview

Fixes #738 (closed).

• Load "Open Sans Bold" font-face explicitly since only some browsers correctly infer the rendering of higher weights. (Chrome & other Chromium browsers on Mac don't appear to do very well, Safari does better but not perfectly.)
• Upgrades font version from v15 to v28 (latest from https://fonts.google.com/specimen/Open+Sans ref. raw css).
• Adds Hebrew character support.

How to Test

• Check out this branch & run Exosphere using npm run live-debug or similar.
• Test case:
  • Log into an instance.
  • Note the font weight of the username in "(logged in as )" & the thickness of text in the Remove Project & Create buttons.
  • Repeat for a selection of browsers.
• Check out another branch like master.
  • Repeat the test case above with a hard reload.

Screenshots

Before

After

Discussion

Now that bold renders correctly, the look of several elements has changed, notably: H1 Exosphere & button text.

Should there be a semi-bold typeface to capture these instead?