Update button on Incident and Alert sidebar widgets

Issue sidebar widget buttons were given new styling at some point, but the Severity widget for incidents was missed. In addition, Alert sidebar widgets are also using the old style.

Example:

Edit hover styling	Dropdown styling

Background: !66165 (comment 695357145)

To Reproduce

For Incidents

1. Create a new incident: Project -> Monitor -> Incidents. Select Create incident.
2. Observe the Severity sidebar widget

For Alerts

1. Configure the Alerts endpoint and trigger a test alert, as per these instructions
2. Navigate to the alerts list: Project -> Monitor -> Alerts
3. Select the test alert.
4. Observe the Status and Assignee widgets.

Proposal

Update the edit button styles for the Severity widget for incidents, and the Alert sidebar widgets.

To align with current styles, the hover behavior on the Edit button needs updating (changing from a link to a button), and the dropdown needs to be changed to a different dropdown variant. For the dropdown, we should be using the header variant, with the text, Assign severity.

Implementation

This could be done by:

• updating the button styles individually to match the design.
• refactoring the widgets to use the existing sidebar-editable-item helper, which provides the button + styling automatically.