Manage bower dependencies with grunt-bowercopy

Tracked bower dependencies are located at "src/sizzle" and "test/libs".
The source-destination mapping is in the Gruntfile.

When updating a bower dependency, update the version in bower.json, run
`grunt bower`, and then commit the result. When adding a dependency,
update the bowercopy task accordingly.

Fixes #14615.
Closes gh-1452.