Rename files to preserve history

Test\Eslint fail - this is expected.

Change-Id: I3a947f259f6316582735d6489aaf3471c4fe21a6
diff --git a/polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.js b/polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.ts
similarity index 100%
rename from polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.js
rename to polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.ts