API endpoint for deleting a file

Depends on #43 (closed), #44 (closed). Subtask of #5 (closed).

changed milestone to %Mailbox: File management API

added Sponsor 6 T: task labels

marked this issue as related to #43 (closed)

marked this issue as related to #44 (closed)

marked this issue as related to #5 (closed)

added Mailbox label

added S: in progress label

assigned to @grote

added Weight: 1 label

mentioned in merge request !28 (merged)

added S: in code review label and removed S: in progress label

mentioned in commit f9ef57bf

closed with merge request !28 (merged)

removed S: in code review label

added R: fixed label

mentioned in issue briar#2233 (closed)

added Mailbox app label