ui: Discovery chain improvements

[johncowen]

This PR includes several minor amends/improvement to the discovery chain visualization:

1. We decided default wasn't the ideal word to use for the final catch-all route/the one that sometimes is added by default - so we reverted this one back to just using Prefix: /
2. Various safer/defensive checks for a few things that we don't think is actually possible (the default router not being able to find a splitter/resolver, the disco-chain component firing an inViewport twice in a row with the same value etc.)
3. We found a very weird templating potential bug where the entire visualisation would flash when appearing (render twice in quick succession), it seemed to be related to the fact that we were using if chain,. I never completely got to the bottom of why this was happening, but found that changing it to if (not-eq chain null) prevented the flashing from happening.
4. As the bug in point 3 ^ sometimes rendered the entire chain twice, we'd occasionally get a bug due to the fact that we were mutating the data and therefore performing the same transformation on already transformed data. So we made the data use a more immutable state where this was happening.

Also took the opportunity to clean up a bit of our whitespace in the template while I was here.

[kaxcode]

LGTM