Spike: GlFilteredSearch: Stop treating spaces specially

This is a spin-off issue based on the discussion in #1438 (comment 906619217), and this (private) recording: https://www.youtube.com/watch?v=FgJBT7iX_6w.

The goal is to time box a proof-of-concept such that GlFilteredSearch no longer treats spaces specially. More concretely, the changes we want to implement are:

• Free text (search terms) must be completed into tokens, via a new “free text” suggestion dropdown.
• Clicking away/out from free text that hasn’t yet been tokenised gets tokenised.
• When typing in the third segment of a token, a new “free text value” option is displayed, allowing for arbitrary tokens containing spaces to be used/searched for
• Pasting text with spaces would behave just like typing them manually (see above)

The goals of the spike would be to:

• Get a better sense of the engineering effort to implement these robustly in GitLab UI
• Get a better sense of the downstream impact in GitLab, in the various places these components are used
• Get a better grasp on how to feature flag this, if we want to start trialling it