Expose severity to display

See https://github.com/EFForg/rayhunter/issues/334

Severity levels low, medium, high are now exposed to the UI in form of
dotted, dashed and solid lines. The line on the UI represents the
highest-so-far severity seen.

Originally this was intended to be represented by Yellow/Orange/Red, but
this would mean yet another divergence for colorblind mode. This is
colorblind-friendly by default (I think...)

As part of this, simplify EventType so that it becomes a flat "level"
enum without nested variants.

There is also a new debug endpoint that allows one to overwrite the
display level directly for testing.

check/src/main.rs

@@ -65,10 +65,10 @@ impl Report {
                 EventType::Informational => {
                     info!("{}: INFO - {} {}", self.file_path, timestamp, event.message,);
                 }
-                EventType::QualitativeWarning { severity } => {
+                EventType::Low | EventType::Medium | EventType::High => {
                     warn!(
                         "{}: WARNING (Severity: {:?}) - {} {}",
-                        self.file_path, severity, timestamp, event.message,
+                        self.file_path, event.event_type, timestamp, event.message,
                     );
                     self.warnings += 1;
                 }