use more performant url join impl

[hulkish]

Summary

• Regex can be heavy handed.

Test plan

yarn test packages/server/src/util.test.js
yarn run v1.16.0
$ jest packages/server/src/util.test.js
 PASS  packages/server/src/util.test.js
  util
    #joinURLPath
      ✓ should join paths with relative public path (2ms)
      ✓ should join paths starting with "/" (1ms)
      ✓ should join paths with absolute public path
      ✓ should join paths with protocol free public path

Test Suites: 1 passed, 1 total
Tests:       4 passed, 4 total
Snapshots:   0 total
Time:        0.869s, estimated 1s
Ran all test suites matching /packages\/server\/src\/util.test.js/i.
✨  Done in 2.02s.

[gregberge]

Thanks!