Allow links to work in custom review request action drop-downs.

Custom review request actions in drop-downs weren't able to link to other
pages, due to the drop-down menu's action handler preventing events from
propagating.

We now only prevent propagation for the menu label itself.

Testing Done:
Testing was only done on the review_export extension and all core menu functionality.

1. Checked other dropdown menus to see if their actions worked (Close -> Submitted, Close -> Discarded)
2. Tried the primary menu actions (actions with no dropdowns)
    a) View Diff
    b) Download Diff
    c) Review
    d) Ship It!
3. Tested the functionality on File Attachment actions:
    a) Update
    b) Delete
All Worked successfully.

Reviewed at https://reviews.reviewboard.org/r/6669/