Faster blog and docs on phones

Changes

• The Japanese typeface (IBM Plex Sans JP) is split into about 120 files by character range, and the blog index loaded more than 60 of them. Each new post added more kanji, so the page was getting slowly slower.
• The blog and docs now use a version of the typeface that contains only the characters in the posts and the interface text. Each weight needs just a few files, and the blog index now loads 7 font files. The letterforms are the same as before.
• Code for parts you don't need until you open them, such as the sign-in and account settings dialogs and the practice celebrations, now loads after the page appears. The blog index loads about a quarter less code up front.
• Characters that aren't in it, such as names you type, are drawn in your device's font.