Make the Chrome Web Store ID the single canonical extension ID

The extension is now published (ID: mkiffclapimhbjjlgkndlillglopbcic).
Drop the pinned "key" field from manifest.json (the Store rejects it
anyway) and use the Store-assigned ID everywhere instead of juggling
two IDs: native-host/com.kget_capture.host.json's allowed_origins,
keys/extension-id.txt, and docs. This also removes the need for a
separate key-stripped release zip, since there's no key left to strip.

native-host/com.kget_capture.host.json

@@ -3,5 +3,5 @@
   "description": "Native messaging host that hands off download URLs to KGet via D-Bus",
   "path": "__HOST_SCRIPT_PATH__",
   "type": "stdio",
-  "allowed_origins": ["chrome-extension://lgkhlfhejbffjnpcdlpjclicihnbjijp/"]
+  "allowed_origins": ["chrome-extension://mkiffclapimhbjjlgkndlillglopbcic/"]
 }