Improving naming scheme for Compare Git Revision

The from / target / start_ref vs to / source / head_ref correspondence remains confusing

Current (confusing!)	Suggestion (better!)

The naming of this is super confusing as mentioned by @nick.thomas > !52967 (comment 501807906). When I was working on this, it also felt super counter-intuitive that we referred "target" as "from" and "source" as "to". Let's clear this muddy water! And adopt a better naming scheme and just use "Source" and "Target" in both the backend and frontend 🙌

Example (frontend)

-  params-name="from"
+  params-name="source"

besides the vue components, we'll probably also need to update the feature specs