Support blank space naming for filter dropdown
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Currently adding spaces in the names of filter options is not possible. The solution thus far has been to add dashes `-` where spaces would exist. But this impairs readability and scanability of the filters list :slight_frown:

### Considerations
1. Using spaces today in the filter bar breaks the query ([example video](https://gitlab.com/gitlab-org/gitlab/uploads/351b9d3635e7d6cc8a4d6260279cc59f/Kapture_2020-04-30_at_17.45.41.webm))
1. Make sure that old URLs, using the current `-` format still work.
### Relevant Files
**The Magic 🎩**
- https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/filtered_search/filtered_search_dropdown_manager.js#L160
- https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/filtered_search/filtered_search_token_keys.js#L52
- https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/filtered_search/filtered_search_manager.js#L734
**CE Labels using dashes**
- "my-reaction" > https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/filtered_search/available_dropdown_mappings.js#L104
- "target-branch" > https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/filtered_search/available_dropdown_mappings.js#L138
**EE Labels using dashes**
- "approved-by" > (not yet merged)
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/30335
issue