Update scripts and docs to use yarn where appropriate

Most `npm` operations are replaced with `yarn`, which generally has better
behavior. However, steps like publish that write to the NPM registry are left to
`npm`, which currently handles these tasks best.

jenkins.sh

@@ -9,24 +9,24 @@ nvm use 10
 set -x
 
 # install the other dependencies
-npm install
+yarn install
 
 scripts/fetchdep.sh matrix-org matrix-js-sdk
 rm -r node_modules/matrix-js-sdk || true
 ln -s ../matrix-js-sdk node_modules/matrix-js-sdk
-(cd matrix-js-sdk && npm install)
+(cd matrix-js-sdk && yarn install)
 
 # run the mocha tests
-npm run test -- --no-colors
+yarn test --no-colors
 
 # run eslint
-npm run lintall -- -f checkstyle -o eslint.xml || true
+yarn lintall -f checkstyle -o eslint.xml || true
 
 # re-run the linter, excluding any files known to have errors or warnings.
-npm run lintwithexclusions
+yarn lintwithexclusions
 
 # delete the old tarball, if it exists
 rm -f matrix-react-sdk-*.tgz
 
 # build our tarball
-npm pack
+yarn pack