About: edit markdown widgets

[e18r]

On the About app, add the ability to edit markdown widgets by adding a hover button right next to the delete button:

When clicked, a side panel will appear that looks identical to the "New widget" side panel... ...except for the following:

• The title is "Edit widget"
• The "Column", "Widget" and "Source" dropdowns don't exist (i. e. none of the dropdowns exist)
• The widget's markdown text shows up in the Markdown text area.

When submitted, a new transaction is created and the selected widget is modified accordingly.

[e18r]

Removing the design label and unassigning @javieralaves as I got a green light from him