Record prominence reason on the event when editing through "Edit metadata"

[garethrees]

During the first manual transparency report we tried to tally the reasons which requests were hidden. In doing so we noticed that when requests have their prominence changed through the "Edit metadata" form no :reason param is recorded in the InfoRequestEvent. See mysociety/whatdotheyknow-theme#922 (comment).

To create a better audit trail when editing prominence in this way we could check whether the :prominence attribute was changed, and if so, redirect to an interstitial where a prominence reason is requested. This can then get added to the edit event that gets created.

EDIT: An extra form field reveal is a better approach #6790 (comment).

[garethrees]

A subtlety here is that when we record the reason through the "hide and notify" action it's set to a request status ("not_foi" or "vexatious") rather than free text. Might want to use some sort of canned reasons a la #6487.

[garethrees]

Adding an InfoRequest#prominence_reason is a better way of handling the form field (#6746). This issue now just exists to remember to record the form field's value when creating the event.

[garethrees]

Related to ensuring it's always recorded on change #7214.

[garethrees]

Closing in favour of #6746 (comment)