feat(GlFilteredSearch): Terms as tokens

This implements the new `termsAsTokens` prop without breaking any
existing tests (it is opt-in).

The next commit will add tests for the new feature. They were not
included in this commit for two reasons:

- This commit has no (major) test changes, and so the passing test suite
  proves that no existing behaviour is changed/broken by the new prop.
- It (hopefully) makes reviewing this easier.

Addresses #2159.