Mask saved request header values until focused

[tomschlick]

Summary

• Mask request header values on the monitor form after they are set, and reveal them when the field is focused.
• Cover the edit form markup so the masking class stays attached to the headers field.

Test plan

• Open a monitor with saved request headers and confirm the values render as dots.
• Click a header value and confirm the text is visible while the field is focused, then masked again after blur.
• Add a new header, type a value, and confirm it stays readable while typing and masks after leaving the field.
• Save the monitor and confirm the header values are unchanged.