Display edit button in the addon page

[javier-godoy]

As an author

I would like to see an an "Edit" button in the addon page, and navigate back to the addon after saving changes.

So that

• I can enter edition from the addon instead of going back through "My components"
• I have a one-click way of entering edition when I notice an error.
• I'm able to observe how the changes look after editing (and fix them in case I entered something wrong).
• I can easily correct the compatibility matrix, which involves editing different versions of the addon.

[samie]

Labeled this as a bug. The "edit"-button should be visible there, if you are logged in. Navigating back might not work correctly though. Need to check that.