application-colors have new keys that may be added via the getter (#23493)

* application-colors have new keys that may be added via the getter

Some colors previously had multiple usage contexts. For example, `accent1` was used both in charts and other parts of
the summarize UI. Now, those notions are being separated, so `accent1` remains a valid key, and `summarize` is a new
valid key. To make sure behaviour remains the same for existing whitelabel users who may have set these keys, when
such a key exists, it is 'split' by the getter.

For example, if the existing application-colors json contains `accent1`, the getter will add a new key `summarize`
with the same value as `accent1`, but only if `accent1` already exists, otherwise it does nothing. This is also true
for keys `brand`, which adds `accent0`, and `accent7`, which adds `filter`

* Make application colors getter make change only once

* Premium feature flag for test